regex: Fix unicode othercasing

regex: Fix unicode othercasing

The old _pcre_ucp_othercase() function was wrong in returning
NOTACHAR (0xffffffff) for characters that aren't changed by upper-
and lower-casing. This led to PCRE internally using incorrect (or
at least inefficient) character classes when using G_REGEX_CASELESS.

E.g. [Z-\x{100}] turned into:

[Z\x{fffe}\x{fffe}\x{fffe}\x{fffe}\x{fffe}\x{fffe}\x{fffe}\x{fffe}\x{fffe}\x{fffe}\x{fffe}\x{fffe}\x{fffe}\x{fffe}\x{fffe}\x{fffe}\x{fffe}\x{fffe}\x{fffe}\x{fffe}\x{fffe}\x{fffe}\x{fffe}\x{fffe}\x{fffe}\x{fffe}\x{fffe}\x{fffe}\x{fffe}\x{fffe}\x{fffe}\x{fffe}\x{fffe}\x{fffe}\x{fffe}\x{fffe}\x{fffe}\x{fffe}\x{fffe}\x{fffe}\x{fffe}\x{fffe}\x{fffe}\x{fffe}\x{fffe}\x{fffe}\x{fffe}\x{fffe}\x{fffe}\x{fffe}\x{fffe}\x{fffe}\x{fffe}\x{fffe}\x{fffe}\x{fffe}\x{fffe}\x{fffe}\x{39c}\x{fffe}\x{fffe}\x{fffe}\x{fffe}\x{fffe}\x{fffe}\x{fffe}\x{fffe}\x{fffe}\x{fffe}\x{fffe}\x{fffe}\x{fffe}\x{178}z-\x{101}]

instead of the expected and efficient

[Z\x{39c}\x{178}z-\x{101}]

https://bugzilla.gnome.org/show_bug.cgi?id=678273