Rewrite translated code to use the pcsc crate

This commit contains a "big bang" refactor/rewrite which does the
following:

- Replaces all `SCard*` FFI calls with the `pcsc` crate, which provides
  a safe, portable PC/SC API across Windows, macOS, and Linux
- Refactors the `util` module into modules representing the various
  device functions and concepts, e.g. `certificate`, `key`, `mgm`
- Replaces all usage of `libc` with `std` functionality, and in many
  places rewriting functionality to use safe code.
- Removes `ykpiv_` from all function names, and `Piv*` from type names.

In 20/20 hindsight I wish I had done this commit more incrementally so
as to make it easier to review. Que sera sera.

However, realistically we need to test all functionality on the device
to ensure that it actually works. Going forward I would like to put
pretty much all of the current code behind an `untested` cargo feature,
and then remove it for each bit of functionality we test.

src/settings.rs

@@ -0,0 +1,138 @@
+//! Configuration setting values parsed from the environment and config file:
+//! `/etc/yubico/yubikeypiv.conf`
+
+// Adapted from yubico-piv-tool:
+// <https://github.com/Yubico/yubico-piv-tool/>
+//
+// Copyright (c) 2014-2016 Yubico AB
+// All rights reserved.
+//
+// Redistribution and use in source and binary forms, with or without
+// modification, are permitted provided that the following conditions are
+// met:
+//
+//   * Redistributions of source code must retain the above copyright
+//     notice, this list of conditions and the following disclaimer.
+//
+//   * Redistributions in binary form must reproduce the above
+//     copyright notice, this list of conditions and the following
+//     disclaimer in the documentation and/or other materials provided
+//     with the distribution.
+//
+// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+// "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+// LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+// A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+// OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+// LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+// DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+// THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+// (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+// OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+
+/// Default location of the YubiKey PIV configuration file
+pub const DEFAULT_CONFIG_FILE: &str = "/etc/yubico/yubikeypiv.conf";
+
+use std::{
+    env,
+    fs::File,
+    io::{BufRead, BufReader},
+};
+
+/// Source of how a setting was configured
+#[derive(Clone, Copy, Debug, Eq, PartialEq)]
+pub enum Source {
+    /// User-specified setting
+    User,
+
+    /// Admin-specified setting
+    Admin,
+
+    /// Default setting
+    Default,
+}
+
+/// Setting booleans
+#[derive(Copy, Clone, Debug)]
+pub struct BoolValue {
+    /// Boolean value
+    pub value: bool,
+
+    /// Source of the configuration setting (user/admin/default)
+    pub source: Source,
+}
+
+impl BoolValue {
+    /// Get a [`BoolValue`] value
+    pub fn get(key: &str, def: bool) -> Self {
+        let mut setting = get_setting_from_file(key);
+
+        if setting.source == Source::Default {
+            setting = get_setting_from_env(key);
+        }
+
+        if setting.source == Source::Default {
+            setting.value = def;
+        }
+
+        setting
+    }
+}
+
+/// Get a boolean config value
+fn get_setting_from_file(key: &str) -> BoolValue {
+    let mut setting: BoolValue = BoolValue {
+        value: false,
+        source: Source::Default,
+    };
+
+    let file = match File::open(DEFAULT_CONFIG_FILE) {
+        Ok(f) => f,
+        Err(_) => return setting,
+    };
+
+    for line in BufReader::new(file).lines() {
+        let line = match line {
+            Ok(line) => line,
+            _ => continue,
+        };
+
+        if line.starts_with('#') || line.starts_with('\r') || line.starts_with('\n') {
+            continue;
+        }
+
+        let (name, value) = {
+            let mut parts = line.splitn(1, '=');
+            let name = parts.next();
+            let value = parts.next();
+            match (name, value, parts.next()) {
+                (Some(name), Some(value), None) => (name.trim(), value.trim()),
+                _ => continue,
+            }
+        };
+
+        if name == key {
+            setting.source = Source::Admin;
+            setting.value = value == "1" || value == "true";
+            break;
+        }
+    }
+
+    setting
+}
+
+/// Get a setting boolean from an environment variable
+fn get_setting_from_env(key: &str) -> BoolValue {
+    let mut setting: BoolValue = BoolValue {
+        value: false,
+        source: Source::Default,
+    };
+
+    if let Ok(value) = env::var(format!("YUBIKEY_PIV_{}", key)) {
+        setting.source = Source::User;
+        setting.value = value == "1" || value == "true";
+    }
+
+    setting
+}