fix shm_open wrongly being cancellable

overhaul sem_open

this function was overly complicated and not even obviously correct.
avoid using openat/linkat just like in shm_open, and instead expand
pathname using code shared with shm_open. remove bogus (and dangerous,
with priorities) use of spinlocks.

this commit also heavily streamlines the code and ensures there are no
failure cases that can happen after a new semaphore has been created
in the filesystem, since that case is unreportable.

clean up, bugfixes, and general improvement for shm_open/shm_unlink

1. don't make non-cloexec file descriptors
2. cancellation safety (cleanup handlers were missing, now unneeded)
3. share name validation/mapping code between open/unlink functions
4. avoid wasteful/slow syscalls

implement POSIX shared memory