explain bumping locklevel in pshared locks

[kib]

Summary

libthr: fix __thr_pshared_offpage() to not use cancellable close()

__thr_pshared_offpage() is used by pthread functions that must not be
cancellation points according to POSIX.  Use direct __sys_close() instead
of interposed close() there.

Fixes:  1bdbd705993e ("Implement process-shared locks support ...")


libthr: explain why pshared lock bumps locklevel