GET_CONFIG:  Get configuration value for key.  Useful so
that other applications can request configuration
information from GDM.  Any key defined as GDM_KEY_*
in gdm.h is supported.
Supported since: 2.6.0.9
Arguments:  <key>
Answers:
OK <value>
ERROR <err number> <english error description>
0 = Not implemented
50 = Unsupported key
200 = Too many messages
999 = Unknown error
