* cygheap.h (cwdstuff): Convert to class. Make posix and dir private.
(cwdstuff::get_posix): New method.
(cwdstuff::reset_posix): New method.
* dcrt0.cc (dll_crt0_1): Call setlocale rather than _setlocale_r.
* environ.cc (environ_init): Ditto. Prefer "C" locale over current
codepage default locale.
* path.cc (chdir): Use cwdstuff::get_posix method instead of accessing
cwdstuff::posix directly.
(cwdstuff::set): Defer creating posix path to first usage.
(cwdstuff::get_posix): Create posix path if it's empty, and return it.
(cwdstuff::get): Create posix path if it's empty.
* strfuncs.cc (sys_cp_wcstombs): Use UTF-8 conversion in the "C"
locale.
(sys_cp_mbstowcs): Ditto.
* syscalls.cc (gen_full_path_at): Fetch CWD posix path locked.
(setlocale): Implement here. Reset CWD posix path.