localedef: Replace avl tree (cddl) with RB tree, plus ...

localedef: Replace avl tree (cddl) with RB tree, plus ...

When FreeBSD adopted DragonFly's locales, the AVL tree code was replaced
with an RB tree equivalent. While cddl wasn't an issue here, let's bring
in FreeBSD's changes since they did the work.

Some other changes:
 * replace (safe) strcpy+strncat with snprintf
 * rework charmap types/prototypes to remove GCC pragmas
 * Support case where wchar_t is an unsigned int (ARM)
 * Change -D "DragonFly-style" option description to -D "BSD-style"