When pushing traffic with iperf the UE was crashing.

The argument passed to pthread_mutex_timedlock is absolute time based
on the CLOCK_REALTIME timer, not relative time. That seems to be the
root cause of the bug. (That there is a timeout is another story.)

This commit uses absolute value.

Also we separate cases between IS_SOFTMODEM_BASICSIM / IS_SOFTMODEM_RFSIM
and standard realtime UE. In the 'sim' modes we don't care about
timeout and just call pthread_mutex_lock. It seems to be the idea
of the original work. If not, then to be fixed.