Return -ENOSYS if PR_[SG]ET_OPENAT_BENEATH missing
Return -ENOSYS if PR_[SG]ET_OPENAT_BENEATH missing

This allows application code to use cap_getmode() as a way
of determining at runtime if capability mode is available:
if cap_getmode() returns -1 with errno ENOSYS then it isn't.

google/capsicum-test,google/capsicum-test,google/capsicum-test,google/capsicum-test