[MEDIUM] add a maintenance mode to servers

[MEDIUM] add a maintenance mode to servers

This is a first attempt to add a maintenance mode on servers, using
the stat socket (in admin level).

It can be done with the following command :
   - disable server <backend>/<server>
   - enable  server <backend>/<server>

In this mode, no more checks will be performed on the server and it
will be marked as a special DOWN state (MAINT).

If some servers were tracking it, they'll go DOWN until the server
leaves the maintenance mode. The stats page and the CSV export also
display this special state.

This can be used to disable the server in haproxy before doing some
operations on this server itself. This is a good complement to the
"http-check disable-on-404" keyword and works in TCP mode.