The mutex attribute now keeps its protocol and priority ceiling the way
FreeBSD's libthr does: the ceiling can be set and read only under
PTHREAD_PRIO_PROTECT (EINVAL otherwise), and setting a protocol resets it
to the highest SCHED_RR priority. Mutexes still do not apply priority
protection; that is listed as a silent stub.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
(cherry picked from commit d10c024f8fdbcdfced0187550f272333264f07df)