Check for available disk space on each pass through the main loop,
so as to handle tasks that consume space quickly on output.
Display space error in units of megabytes.

Conflicts:
	dttools/src/work_queue_worker.c
Fixed parsing to be consistent with help message: disk space limit is expressed as plain number without units.
Previous code accepted units but didn't handle them correctly.
Add debug msg when resetting execv syscall number.