[MEDIUM] enable/disable servers from the stats web interface

[MEDIUM] enable/disable servers from the stats web interface

Based on a patch provided by Judd Montgomery, it is now possible to
enable/disable servers from the stats web interface. This allows to select
several servers in a backend and apply the action to them at the same time.

Currently, there are 2 known limitations :
- The POST data are limited to one packet
  (don't alter too many servers at a time).
- Expect: 100-continue is not supported.
(cherry picked from commit 7693948766cb5647ac03b48e782cfee2b1f14491)