Given the 64-bit timebase, add trylock_until to Mutex.

Naming is based on a combination of Mutex::trylock, Thread::wait_until,
and C++11 timed_mutex::try_lock_until.

pthreads and C11 use "timedlock", but that's not a good fit against our
existing trylock() and lock(timeout) - they have only absolute-time
waits, not relative.

To increase the similarity to C++11, add trylock_for - same parameters
as lock, but with the bool return value of trylock and trylock_until.

Add an assertion when convering status codes to booleans to check that
there are no non-timeout errors.