mserver5: do a slight check on debug argument
mserver5: do a slight check on debug argument

Make sure debug is a number such that wrong input doesn't go unnoticed
when we do something like mserver5 --debug=HEADLESS (obviously, this was
too naive, but anyway).

zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb