feat(doctor): check the named distro, staged agent, hvsocket registration and the port

[zcsizmadia]

Closes #77.

wsldrive doctor checked WinFsp, wsl.exe and the default distro. That left it unable to answer the questions people actually arrive with — including the one the Direction B setup notes explicitly send them to doctor for.

doctor [--distro X] [--hvsocket] [--port N] [--pause]

check	why
--distro X runs a command in that distro	mount --distro X otherwise fails later, for its own reasons, on a name that was simply wrong
staged ~/.local/bin/wsldrived	a mount that auto-launches the agent fails at launch, not at configure. A warning, not a failure: --agent can point elsewhere
--hvsocket counts the GuestCommunicationServices GUIDs	the check Direction B users are sent here for, which doctor could not answer
--port N binds the loopback port	a stale agent holding it is named now, instead of surfacing later as a confusing mount failure
--pause holds the console open	the Inno postinstall ran doctor in a window that closed before the report could be read; it now passes --pause

Unknown options are refused with exit 2 rather than ignored.

It found a real problem on the first run. The ten hvsocket registrations this machine was set up with are gone — 0/10 — which I confirmed independently against the registry before trusting the check, so it is a true negative and not a wrong key path.

Verification

doctor                                   -> all ok, exit 0
doctor --distro Ubuntu --hvsocket --port 5700
  [ok]   distro 'Ubuntu' runs commands
  [ok]   agent staged in Ubuntu (~/.local/bin/wsldrived)
  [fail] only 0/10 hvsocket GUIDs registered for ports 5700-5709
  [ok]   TCP port 5700 is free      -> exit 1
doctor --distro NoSuchDistro             -> distro check fails, agent check warns
doctor --bogus                           -> exit 2

The wizard still compiles. Windows-only code, so Linux is untouched: 126/126 and conformance 31/0 there; Windows 124/124.