increase limit on locale name length from 15 to 23 bytes

the old limit was one byte too short to support locale names of the
form xx_XX.UTF-8@modifier where modifier is more than 3 bytes, a form
which various real-world locale names take. the problem could be
avoided by omitting the useless ".UTF-8" part, but users may need to
have it present when operating on mixed-libc systems or when it will
be carried over (e.g. across ssh) to other systems.

the new limit is chosen sufficient for existing/reasonable locale
names while still keeping the size of setlocale's static buffer small.

also add locale_impl.h to the Makefile's list of headers which force
rebuild of source files, to prevent dangerously inconsistent object
files from getting used after this change.
2 files changed