Cygwin: //server: revert to using WNet and support NFS shares

Cygwin: //server: revert to using WNet and support NFS shares

Continue using WSD for enumerating //, but switch back to
WNet for enumerating shares.

Add trying to enumerate NFS shares for machine names given as FQDN.

Only downcase server names, keep case of shares intact.
Move downcasing into dir_cache class.

Add a comment to explain an extrem weirdness in Windows.

Fixes: 205190a80bd0 ("Cygwin: // and //server: switch to Network Discovery")
Signed-off-by: Corinna Vinschen <corinna@vinschen.de>