Make -Pn override all other -P* types

This was a case where order of arguments affected the outcome: if -PE
came before -Pn, then -Pn took precedence. Otherwise, -PE took
precedence (except that o.pingtype would also contain PINGTYPE_NONE...
not sure how that affected things). This cleans things up by letting
PINGTYPE_NONE be OR'd into o.opingtype, then checking for it after all
options have been processed and clearing out the other types if it is
found.