QUERY_VT:  Ask the daemon about which VT we are currently on.
This is useful for logins which don't own
/dev/console but are still console logins.  Only
supported on Linux currently, other places will
just get ERROR 8.  This is also the way to query
if VT support is available in the daemon in the
first place.
Only supported on connections that passed AUTH_LOCAL.
Supported since: 2.5.90.0
Arguments:  None
Answers:
OK <vt number>
ERROR <err number> <english error description>
0 = Not implemented
8 = Virtual terminals not supported
100 = Not authenticated
200 = Too many messages
999 = Unknown error
