ALL_SERVERS: List all servers, including console, remote, xnest.
This can, for example, be useful to figure out if
the server you are on is managed by the gdm daemon,
by seeing if it is in the list.  It is also somewhat
like the 'w' command but for graphical sessions.
Supported since: 2.4.2.96
Arguments:  None
Answers:
OK <server>;<server>;...
<server> is <display>,<logged in user>
<logged in user> can be empty in case no one logged in yet
ERROR <err number> <english error description>
0 = Not implemented
200 = Too many messages
999 = Unknown error
