`pthread_mutex_trylock` return `0` on success.
```
NAME
pthread_mutex_trylock – attempt to lock a mutex without blocking
SYNOPSIS
#include <pthread.h>
int
pthread_mutex_trylock(pthread_mutex_t *mutex);
DESCRIPTION
The pthread_mutex_trylock() function locks mutex. If the mutex is already locked,
pthread_mutex_trylock() will not block waiting for the mutex, but will return an error
condition.
RETURN VALUES
If successful, pthread_mutex_trylock() will return zero, otherwise an error number will
be returned to indicate the error.
ERRORS
The pthread_mutex_trylock() function will fail if:
[EINVAL] The value specified by mutex is invalid.
[EBUSY] Mutex is already locked.
```
There has been some discussion as to whether this is the right approach,
centered around internal pointers in the underlying mutex structure.
After much thought, let's commit this for now. The argument:
* While it is true that OpenBSD has futex(2) and in fact is used in
the underlying implementations in Dispatch. I'm not confident right now
creating robust locking primitives by hand from futexes.
It would be ideal to just delegate all of this to Dispatch, but
Dispatch doesn't expose a ready-use API for mutexes. Cobbling one
together might be possible, but that would take some time and finesse.
* While recent versions of C and C++ have language support for mutexes,
they are implemented in many cases by libpthread anyway, so there
is nothing inherently gained right now by not using the pthread API
other than portability.
* While the concern for internal pointers for the pthread API is
important, the OpenBSD is mitigated by the fact that pthread_mutex_t is
pointer-valued on the platform. The concern that this might not always
be the case is mitigated by the fact that this commit does not attempt
to use the pthread implementation as a catch-all for all platforms.
* The alternative is to use MutexUnavailable, but given Foundation has
some dependencies on Mutex, this may cause problems elsewhere
throughout the port.
