MEDIUM: server: move _srv_check_proxy_mode() checks from server init to finalize

MEDIUM: server: move _srv_check_proxy_mode() checks from server init to finalize

_srv_check_proxy_mode() is currently executed during server init (from
_srv_parse_init()), while it used to be fine for current checks, it
seems it occurs a bit too early to be usable for some checks that depend
on server keywords to be evaluated for instance.

As such, to make _srv_check_proxy_mode() more relevant and be extended
with additional checks in the future, let's call it later during server
finalization, once all server keywords were evaluated.

No change of behavior is expected