Make subword.el easier to customize (tiny change)

Make subword.el easier to customize (tiny change)

* lisp/progmodes/subword.el (subword-forward-function)
(subword-backward-function, subword-forward-regexp, subword-backward-regexp):
New variables.
(subword-forward, subword-forward-internal, subword-backward-internal):
Use new variables, eg so that different "word" definitions can be easily used.

Fixes: debbugs:11411