ATTACHED_SERVERS: List all attached servers.  Doesn't list XDMCP and xnest
non-attached servers
Supported since: 2.2.4.0
Note: This command used to be named CONSOLE_SERVERS, which is still recognized
for backwards compatibility. The optional pattern argument is supported
as of version 2.8.0.0.
Arguments:  <pattern> (optional)
With no argument, all attached displays are returned. The optional
<pattern> is a string that may contain glob characters '*', '?', and
'[]'. Only displays that match the pattern will be returned.
Answers:
OK <server>;<server>;...
<server> is <display>,<logged in user>,<vt or xnest display>
<logged in user> can be empty in case no one logged
in yet, and <vt> can be -1 if it's not known or not
supported (on non-Linux for example).  If the display is an
xnest display and is a console one (that is, it is an xnest
inside another console display) it is listed and instead of
vt, it lists the parent display in standard form.
ERROR <err number> <english error description>
1 = Not implemented
200 = Too many messages
999 = Unknown error
