BUG/MINOR: server-state: Don't load server-state file for disabled backends

BUG/MINOR: server-state: Don't load server-state file for disabled backends

Recent changes on the server-state file loading have introduced a
regression. HAproxy crashes if a backend with no server-state file is
disabled in the configuration. Indeed, configuration of such backends is not
finalized. Thus many fields are not defined.

To fix the bug, disabled backends must be ignored. In addition a BUG_ON()
has been added to verify the proxy mode regarding the server-state file. It
must be specified (none, global or local) for enabled backends.

No backport needed.