MINOR: checks: use an enum instead of flags to report a check result

MINOR: checks: use an enum instead of flags to report a check result

We used to have up to 4 sets of flags which were almost all exclusive
to report a check result. And the names were inherited from the old
server states, adding to the confusion. Let's replace that with an
enum handling only the possible combinations :

   SRV_CHK_UNKNOWN                   => CHK_RES_UNKNOWN
   SRV_CHK_FAILED                    => CHK_RES_FAILED
   SRV_CHK_PASSED                    => CHK_RES_PASSED
   SRV_CHK_PASSED | SRV_CHK_DISABLE  => CHK_RES_CONDPASS