MFC r333622, r334019, r334084

r333622:
vq_getchain() can return -1 if some descriptor(s) are invalid and prints
a diagnostic message. So we do a sanity checking on the return value
of vq_getchain().

Spotted by: gcc49
Reviewed by: avg
Sponsored by: iXsystems Inc.
Differential Revision: https://reviews.freebsd.org/D15388

r334019:
Include atkbdc header where there are declared the prototype functions
atkbdc_event and atkbdc_init.

Sponsored by: iXsystems Inc.

r334084:
pthread_rwlock_unlock(3) returns 0 if successful, otherwise an error number
will be returned to indicate the error, so I'm applying an assert(3) to do
a sanity check of the return value.

Reported by: Coverity CID: 1391235, 1193654 and 1193651
Reviewed by: grehan
Sponsored by: iXsystems Inc.
Differential Revision: https://reviews.freebsd.org/D15533