handy.h: Create isALPHANUMERIC() and kin

handy.h: Create isALPHANUMERIC() and kin

Perl has had an undocumented macro isALNUMC() for a long time.  I want
to document it, but the name is very obscure.  Neither Yves nor I are
sure what it is.  My best guess is "C's alnum".  It corresponds to
/[[:alnum:]]/, and so its best name would be isALNUM().  But that is the
name long given to what matches \w.  A new synonym, isWORDCHAR(), has
been in place for several releases for that, but the old isALNUM()
should remain for backwards compatibility.

I don't think that the name isALNUMC() should be published, as it is too
close to isALNUM().  I finally came to the conclusion that
isALPHANUMERIC() is the best name; it describes its purpose clearly; the
disadvantage is its long length.  I doubt that it will get much use, but
we need something, I think, that we can publish to accomplish this
functionality.

This commit also converts core uses of isALNUMC to isALPHANUMERIC.  (I
intended to that separately, but made a mistake in rebasing, and
combined the two patches; and it seemed like not a big enough problem to
separate them out again.)