Name: set_heart_beat - enable or disable heart beat.

Syntax:
	int set_heart_beat(int flag)

Description:
	If the heart beat is not needed for the moment, then do disable it.
	This will reduce system over head.

Return value:
	Return true for success, and false for failure. Specifically, it
	will fail if the heart beat function has been disabled, which it
	will be, if there is a run time error in it.

See also: object/heart_beat, efun/call_out