Implement POSIX-2008 locale_t APIs

[jessicah]

Tracking for: POSIX-2008 introduces a thread-local locale API.

Ref: ​https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap07.html

A new locale_t type, various additions to <ctype.h> and <locale.h>.

Came across this being needed for an up to date GHC port.

[pulkomandy]

I'm moving this to the unscheduled milestone unless there are active plans to work on it soon. I find having a lot of tickets in the current milestone makes it hard to know where we're going with the next release?

[ivan_holmes]

Just to add, I've also run into this recently while trying to port an up-to-date version of fish shell (although I patched out locale_t and it still doesn't work for other reasons).

[waddlesplash]

Implemented in hrev56270.