MEDIUM: server: split srv_init() in srv_preinit() + srv_postinit()

MEDIUM: server: split srv_init() in srv_preinit() + srv_postinit()

We actually need more granularity to split srv postparsing init tasks:
Some of them are required to be run BEFORE the config is checked, and
some of them AFTER the config is checked.

Thus we push the logic from 368d0136 ("MEDIUM: server: add and use
srv_init() function") a little bit further and split the function
in two distinct ones, one of them executed under check_config_validity()
and the other one using REGISTER_POST_SERVER_CHECK() hook.

SRV_F_CHECKED flag was removed because it is no longer needed,
srv_preinit() is only called once, and so is srv_postinit().