Fix the broken warning function (MWARNING in particular)

Fix the broken warning function (MWARNING in particular)

Calling this function with zero arguments would cause a lisp error,
and calling this function with more than one argument wouldn't work
as expected since it would only print the result from the first
argument.

Perhaps further work can be done to improve warnings messages (for
example, maybe MWARNING could actually take a MFORMAT-style control
string as its first argument), but at least it's not broken now and
I think it matches the original intent of the code (for whatever
that's worth).

The warning and MWARNING functions were introduced in commit
da73a1cb.