This adds a i_wcwidth() function that replaces mk_wcwidth(), and a
'wcwidth_implementation' setting to pick which one it wraps.
Values:
- old: uses our local mk_wcwidth() which implements unicode 5.0
- system: uses the libc-provided wcwidth(), which may be better or worse
than ours depending on how up to date the system is.
- auto: tests the system one against two characters that became
fullwidth in unicode 5.2 and 9.0 respectively. If either of them pass,
pick the system implementation, otherwise pick ours.
It defaults to auto.
mk_wcwidth() is still preferable in some cases, since the way it uses
ranges for fullwidth characters means most CJK blocks are covered even
if their characters didn't exist back then.
The "system" implementation is also wrapped to never return -1, but to
assume those unknown characters use one cell. Quoting the code:
/* Treat all unknown characters as taking one cell. This is
* the reason mk_wcwidth and other outdated implementations
* mostly worked with newer unicode, while glibc's wcwidth
* needs updating to recognize new characters.
*
* Instead of relying on that, we keep the behavior of assuming
* one cell even for glibc's implementation, which is still
* highly accurate and less of a headache overall.
*/
