fix: macOS and Windows test/lint failures; repo.dixon.cx packaging for nexusterminal

CI hadn't run on macOS or Windows for a while (billing); once it did:
- Windows: the RDP shared-drive code and its symlink test are Unix-only
  but were compiled there (and the unused pieces failed clippy). The drive
  type, its impls, imports and test are now cfg(unix); refusing a drive
  request is a free function used everywhere. Other Windows-only warnings
  fixed (serial test import, unused mut, harden_process).
- macOS: the local shell's directory was sampled on the first output
  after Enter, which is the shell echoing the command before a cd runs,
  so restored tabs opened one directory behind. It's now sampled shortly
  after Enter and again once the command has had time to finish.
- packaging/repo.sh publishes the 'nexusterminal' .deb/.rpm/Arch packages
  to repo.dixon.cx from the release's own Linux binaries, verified
  against SHA256SUMS.
- The desktop entry mentions RDP.

rust/core/src/clients/serial.rs

@@ -206,6 +206,7 @@ fn open_error(path: &str, e: &serialport::Error) -> crate::Error {
 #[cfg(test)]
 mod tests {
     use super::*;
+    #[cfg(unix)]
     use std::sync::mpsc;
 
     #[test]