capsicum: require CAP_POLL_EVENT for mq_getsetattr

This behaviour is analogous to FreeBSD.
capsicum-test: update kernelspace tests for changed structure

Internals of capsicum_pending_syscall changed with commit ae535a77dd;
update internal tests to reflect that change.
capsicum-test: remove mmap test

This test is now implemented in userspace in the capsicum-test
repository, so an internal kernel test is no longer needed.
capsicum: temporarily include debugging macro

For development purposes it's nice to be able to quickly drop
in tracing calls, so include a kdebug() macro for this.
capsicum: record current task in fresh capsicum_pending_syscall

The previous code would leave the pending->task field as NULL,
so that when the capsicum_pending_syscall was used it would appear
to be for the wrong task and so would be replaced with a fresh
copy.
capsicum-test: remove tests of cred duplication

This code path is exercised from userspace in the new thread/process
tests in capsicum-test.