regexec.c: Allow for returning shared swash

regexec.c: Allow for returning shared swash

This changes the function that returns the swash associated with a
bracketed character class so that it returns the original swash and not
a copy.  The function is renamed and made accessible only from within
regexec.c, and a new wrapper function with the original name is created
that just calls the other one and returns a copy of the swash.

Thus, all access from outside regexec.c will use a copy which if
overwritten will not harm others; while the option exists from within
regexec.c to use a shared version.