GREETERPIDS: List all greeter pids so that one can send HUP
to them for config rereading.  Of course one
must be root to do that.
Supported since: 2.3.90.2
Arguments:  None
Answers:
OK <pid>;<pid>;...
ERROR <err number> <english error description>
0 = Not implemented
200 = Too many messages
999 = Unknown error
