MINOR: checks: use check->state instead of srv->state & SRV_CHECKED

MINOR: checks: use check->state instead of srv->state & SRV_CHECKED

Having the check state partially stored in the server doesn't help.
Some functions such as srv_getinter() rely on the server being checked
to decide what check frequency to use, instead of relying on the check
being configured. So let's get rid of SRV_CHECKED and SRV_AGENT_CHECKED
and only use the check's states instead.