Cleanup: LocaleBackend.h is not a local header for wchar.

Fix POSIX-specific mbsrtowcs() and wcsrtombs().

* src was sometimes accessed incorrectly (needs double dereference)
* the source pointers may only be adjusted in case there is the
destination pointer is not NULL

Drop public symbols for mbsnrtowcs() and wcsnrtombs().

We only implement POSIX_VERSION 199009, so these needn't be exported.

Add implementation of mbsrtowcs() to our locale backend.

* add MultibyteStringToWchar() to ICU locale backend
* implement mbsrtowcs() and mbsnrtowcs() on top of
MultibyteStringToWchar()
* drop respective glibc files

Cleanup: LocaleBackend.h is not a local header for wchar.

Fix POSIX-specific mbsrtowcs() and wcsrtombs().

* src was sometimes accessed incorrectly (needs double dereference)
* the source pointers may only be adjusted in case there is the
destination pointer is not NULL

Drop public symbols for mbsnrtowcs() and wcsnrtombs().

We only implement POSIX_VERSION 199009, so these needn't be exported.

Add implementation of mbsrtowcs() to our locale backend.

* add MultibyteStringToWchar() to ICU locale backend
* implement mbsrtowcs() and mbsnrtowcs() on top of
MultibyteStringToWchar()
* drop respective glibc files