Make generation numbers catch more potential yields on input

Make generation numbers catch more potential yields on input

getstarg(), snxtitem() and snxtsct() can yield the processor, because
they call getstring().  But only for null or empty arguments.  For
other arguments, we should call ef_make_stale(), to catch errors.
Problem: if a caller never passes null or empty arguments, it may rely
on these functions not yielding.  We'd get false positives.  In
general, we can't know whether that's the case.  But we do know in the
common special case of player arguments.  Call ef_make_stale() for
those.