posix: semaphore: fix bugs and simplify code

Modifies several functions that are causing wrong
behaviour.

 * semaphore.h: add missing restrict keyword.
 * sem_destroy(): check that nobody is waiting
   before destroying the object.
 * sem_timedwait(): simpify function logic and
   fix a bug when abstime > currtime, that passed
   ticks instead of ms to k_sem_take().
 * sem_wait(): avoid unnecessary checks.
 * sem_init(): add pshared value assertion.

Signed-off-by: Juan Manuel Torres Palma <j.m.torrespalma@gmail.com>