add no_exec_limit field. Set to 1 to disable the runaway loop check
for unlimited runs (eg, in qwaq)
pr_exec.c:
don't bother checking the profile counter if pr->no_exec_limit is set
pr_strings.c:
free unreferenced dynamic strings rather than referenced.
external to the progs file are now malloced and then freed at progs reload.
All that needs to be implementd for gc to work is the scanner and deallocator.