sem_timedwait() function

This is a POSIX function, please refer to the POSIX specification for a detailed description. Note the following three deviations/issues originating from the underlying FreeRTOS implementation: * The time value passed by abstime will be rounded up to the next FreeRTOS tick. * The actual timeout will happen after the tick the time was rounded to and before the following tick. * It is possible, though unlikely, that the task is preempted directly after the timeout calculation, delaying timeout of the following blocking operating system call by the duration of the preemption.