g_utf8_make_valid was turned into a public API this cycle. However
now that it is public we should make the API more generic, allowing
the caller to specify the length. This is especially useful if
the function is called with a string that has \0 in the middle
or for chunks of a strings that are not nul terminated.
This is also consistent with most of the other utf8 utils.

Callers inside glib are updated to the new signature.

https://bugzilla.gnome.org/show_bug.cgi?id=779456