Implement throtteling of main server loop if there is a thread for each
core already.

Environment variable CFENGINE_THROTTLE_SERVER_LOOP can be set to an
integer multiplier that allows fine tuning. If not set or set to 0, no
throttling.

Only implemented on platforms where we can trivially read the number of
cores.
Merge remote-tracking branch 'vohi/throttle_serverloop' into 3.3.x-exit_cleanly

Conflicts:
	src/cf-serverd.c
Small fixes