Allow accesses of the caller's CPU and domain sets in capability mode.

cpuset_(get|set)(affinity|domain)(2) permit a get or set of the calling
thread or process' CPU and domain set in capability mode, but only when
the thread or process ID is specified as -1. Extend this to cover the
case where the ID actually matches the caller's TID or PID, since some
code, such as our pthread_attr_get_np() implementation, always provides
an explicit ID.

It was not and still is not permitted to access CPU and domain sets for
other threads in the same process when the process is in capability
mode. This might change in the future.

Submitted by: Greg V <greg@unrelenting.technology> (original version)
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D25552