alling mq_timedreceived with immediate timeout was getting stuck and not

timeout. Immediate timeout is achieved by setting absolute timeout value to
past time, for example abstime={ .tv_sec=0, .tv_nsec=0 }. However absolute
time was converted to relative time using unsigned integer arithmetic and
resulted large ticks count by clock_abstime2ticks, instead of expected negative
ticks value.

Patch corrects clock_abstime2ticks to return negative ticks, if absolute time
is in the past.

Signed-off-by: Jussi Kivilinna <jussi.kivilinna@haltian.com>