MAJOR: checks: add support for a new "drain" administrative mode

MAJOR: checks: add support for a new "drain" administrative mode

This patch adds support for a new "drain" mode. So now we have 3 admin
modes for a server :
  - READY
  - DRAIN
  - MAINT

The drain mode disables load balancing but leaves the server up. It can
coexist with maint, except that maint has precedence. It is also inherited
from tracked servers, so just like maint, it's represented with 2 bits.

New functions were designed to set/clear each flag and to propagate the
changes to tracking servers when relevant, and to log the changes. Existing
functions srv_set_adm_maint() and srv_set_adm_ready() were replaced to make
use of the new functions.

Currently the drain mode is not yet used, however the whole logic was tested
with all combinations of set/clear of both flags in various orders to catch
all corner cases.