use POSIX realtime timers as tick source on Linux and FreeBSD, fallback to pthreads on other systems

raise the timers resolution to 100 msec. sleep() and after() expect the timeout argument to be in msecs,
use the defined Milisecond, Second, Minute, Hour constants as multipliers.

init_timers() must be called as early as possible, before any os thread is created. It will mask the
realtime signal SIGRTMIN+9 on systems with posix realtime extensions.
The rt timer clock is set to CLOCK_MONOTONIC.

The pthreads fallback relies on pthread_cond_timedwait. Not all systems support pthread_condattr_setclock
for setting the pthread_cond_timedwait clock to CLOCK_MONOTONIC, so assume CLOCK_REALTIME is used.