Local typedef of cap_rights_t in Linux
Reduce flakiness of Pdfork.Pdkill test
Add test that pdkill(2) generates killed-by-signal exit status
Mark skipped test when kcmp(2) unavailable
Initialize flags before sctp_recvmsg

This test was flaking sometimes, returning EINVAL rather than
ENOTCAPABLE for sctp_recvmsg() on a not-read-capable socket.
Only ever kill(2) real pids.

If the tests are run on a non-Capsicum system, the pid values
are likely to be -1.  Avoid running kill(-1, SIGKILL)  (which
terminates all the user's processes!).