Dunk on NT and Darwin bc they don't use O_DIRECT which causes serious

problems for this project.

src/cli.rs

@@ -35,6 +35,12 @@ pub struct Args {
     // ArgAction::SetFalse by default evaluates to true,
     // ArgAction::SetTrue by default evaluates to false.
     /// Upon encountering a read error, reopen the source file before continuing.
-    #[arg(short, long, action=ArgAction::SetTrue)]
+    #[arg(short, long, action = ArgAction::SetTrue)]
     pub reopen_on_error: bool,
+
+    /// Use O_DIRECT to bypass kernel buffer when reading.
+    // BSD seems to support O_DIRECT, but MacOS for certain does not.
+    #[cfg(all(unix, not(target_os = "macos")))]
+    #[arg(short, long = "direct", action = ArgAction::SetFalse)]
+    pub direct_io: bool,
 }