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

Simplify wcrtomb() wrapper.

* since the backend now supports it, we can safely pass NULL as s
to the backend

Add tracing to some locale backend functions.

Introduce __set_errno() throughout libroot.

* add errno_private.h, which defines the __set_errno() macro with
and without tracing
* instead of setting errno manually, all libroot's code now invokes
__set_errno(), which makes it much easier to trace changes to errno
* redirect glibc's use of __set_errno() to our own version

Activate our new multibyte implementation.

* add implementations for the following multibyte-related
functions:
btwoc()
mblen()
mbrlen()
mbrtowc()
mbsinit()
mbtowc()
wcrtomb()
wcswidth()
wctob()
wctomb()
* the implementation of the above function live in a symbol
named __<name>, the above symbol names are defined as a weak
alias to the internal ones - TODO: we need to make sure to
only invoked the internal functions (i.e. prepended with __)
in order to avoid problems with symbol preemption.
* deactivate the limited mb implementation we provided before,
as well as respective stuff from glibc

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

Simplify wcrtomb() wrapper.

* since the backend now supports it, we can safely pass NULL as s
to the backend

Add tracing to some locale backend functions.

Introduce __set_errno() throughout libroot.

* add errno_private.h, which defines the __set_errno() macro with
and without tracing
* instead of setting errno manually, all libroot's code now invokes
__set_errno(), which makes it much easier to trace changes to errno
* redirect glibc's use of __set_errno() to our own version

Activate our new multibyte implementation.

* add implementations for the following multibyte-related
functions:
btwoc()
mblen()
mbrlen()
mbrtowc()
mbsinit()
mbtowc()
wcrtomb()
wcswidth()
wctob()
wctomb()
* the implementation of the above function live in a symbol
named __<name>, the above symbol names are defined as a weak
alias to the internal ones - TODO: we need to make sure to
only invoked the internal functions (i.e. prepended with __)
in order to avoid problems with symbol preemption.
* deactivate the limited mb implementation we provided before,
as well as respective stuff from glibc