smp: Fix timeout for MrsP semaphores

The previous timeout handling was flawed.  In case a waiting thread
helped out the owner could use the scheduler node indefinitely long.
Update the resource tree in _MRSP_Timeout() to avoid this issue.

Bug reported by Luca Bonato.
posix: Delete unused _POSIX_Threads_Get()

Close #1759.
Merge remote-tracking branch 'upstream/master'