BUG/MAJOR: can't enable a server through the stat socket

BUG/MAJOR: can't enable a server through the stat socket

When a server is disabled in the configuration using the "disabled"
keyword, a single flag is positionned: SRV_ADMF_CMAINT (use to be
SRV_ADMF_FMAINT)..
That said, when providing the first version of this code, we also
changed the SRV_ADMF_MAINT mask to match any of the possible MAINT
cases: SRV_ADMF_FMAINT, SRV_ADMF_IMAINT, SRV_ADMF_CMAINT

Since SRV_ADMF_CMAINT is never (and is not supposed to be) altered at
run time, once a server has this flag set up, it can never ever be
enabled again using the stats socket.

In order to fix this, we should:
- consider SRV_ADMF_CMAINT as a simple flag to report the state in the
  old configuration file (will be used after a reload to deduce the
  state of the server in a new running process)
- enabling both SRV_ADMF_CMAINT and SRV_ADMF_FMAINT when the keyword
  "disabled" is in use in the configuration
- update the mask SRV_ADMF_MAINT as it was before, to only match
  SRV_ADMF_FMAINT and SRV_ADMF_IMAINT.

The following patch perform the changes above.
It allows fixing the regression without breaking the way the up coming
feature (seamless server state accross reloads) is going to work.

Note: this is 1.6-only, no backport needed.