Named mutex might not value permissions correctly

Hi!

When using a named_mutex within POSIX, and setting the Permissions Object, everything boils down to an ::sem_open call. There, the permissions are passed. But ::sem_open masks the permissions with the processes umask. Therefore, the semaphore might not have 0666 permissions, but e.g. 644, even though, permissions were passed with set_unrestricted().

Workaround there would be setting umask to 0 before ::sem_open, and restoring again after.
At least, this behaviour should be documented.

Regards,

Matthias