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/packaging/linux/nexus-terminal.desktop

@@ -2,10 +2,10 @@
 Type=Application
 Name=Nexus Terminal
 GenericName=Terminal and Remote Connections
-Comment=SSH, Telnet, RAW TCP, Serial, VNC, IRC and GitHub sessions in one window
+Comment=SSH, Telnet, RAW TCP, Serial, VNC, RDP, IRC and GitHub sessions in one window
 Exec=nexus-terminal
 Icon=nexus-terminal
 Terminal=false
 Categories=System;TerminalEmulator;Network;RemoteAccess;
-Keywords=ssh;telnet;serial;sftp;vnc;irc;terminal;
+Keywords=ssh;telnet;serial;sftp;vnc;rdp;irc;terminal;
 StartupWMClass=nexus-terminal