BUG/MINOR: check: Don't perform any check on servers defined in a frontend

BUG/MINOR: check: Don't perform any check on servers defined in a frontend

If a server is defined in a frontend, thus a proxy without the backend
capability, the 'check' and 'agent-check' keywords are ignored. This way, no
check is performed on an ignored server. This avoids a segfault because some
part of the tcpchecks are not fully initialized (or released for frontends
during the post-check).

In addition, an test on the server's proxy capabilities is performed when
checks or agent-checks are initialized and nothing is performed for servers
attached to a non-backend proxy.

This patch should fix the issue #1043. It must be backported as far as 2.2.