add FUTEX_PRIVATE macro to internal futex.h

unify and overhaul timed futex waits

new features:

- FUTEX_WAIT_BITSET op will be used for timed waits if available. this
saves a call to clock_gettime.

- error checking for the timespec struct is now inside __timedwait so
it doesn't need to be duplicated everywhere. cond_timedwait still
needs to duplicate it to avoid unlocking the mutex, though.

- pushing and popping the cancellation handler is delegated to
__timedwait, and cancellable/non-cancellable waits are unified.

initial check-in, version 0.5.0