POSIX Message Queue functions that accept timeout of type CLOCK_MONOTONIC

The POSIX message queue functions:

mq_timedreceive
mq_timedsend

are limitied to using CLOCK_REALTIME as timeout parameter.

Here is an implementation of two wrapper functions for them, named:

mq_timedreceive_monotonic
mq_timedsend_monotonic

that accept as timeout parameter CLOCK_MONOTONIC.

Implementation was done using Linux API function poll()