BUG/MINOR: proxy/server: Skip per-proxy/server post-check for disabled proxies

BUG/MINOR: proxy/server: Skip per-proxy/server post-check for disabled proxies

per-proxy and per-server post-check callback functions must be skipped for
disabled proxies because most of the configuration validity check is skipped for
these proxies.

This patch must be backported as far as 2.1.

(cherry picked from commit d5bd824b81cda4c02e967b05af920f455d4b3712)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>
(cherry picked from commit eb20100fe1be0f2641de331ddcc8b1246cd4db2b)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>