Tweaks to count-words and count-words-region behavior.

Tweaks to count-words and count-words-region behavior.
In particular, make count-words more analogous to the existing
count-lines function.

* lisp/simple.el (count-words): If called from Lisp, return the word
count, for symmetry with `count-lines'.  Arglist changed.
(count-words--message): Args changed.  Consolidate counting code
from count-words and count-words-region.
(count-words-region): Caller changed.
(count-lines-region): Make it an obsolete alias.