This replaces a partial set of non-functional functions with a complete
set of functions, all of which actually work.
This requires us to implement mbsnrtowcs and wcsnrtombs which completes
the set of what we need for libc++.
The mbsnrtowcs is basically a copy & paste of wcsnrtombs, but I'm going
to go straight to looking at using the OpenBSD UTF-8 implementation rather
than keep polishing our home-grown turd.
(This patch also opportunistically switches us over to upstream btowc,
mbrlen, and wctob, since they're all trivially expressed in terms of
other functions.)
Change-Id: I0f81443840de0f1aa73b96f0b51988976793a323
3d7a0d9b08