Consolidate wctomb/mbtowc calls for POSIX-1.2008

- Remove charset parameter from low level __foo_wctomb/__foo_mbtowc calls.
- Instead, create array of function for ISO and Windows codepages to point
  to function which does not require to evaluate the charset string on
  each call.  Create matching helper functions.  I.e., __iso_wctomb,
  __iso_mbtowc, __cp_wctomb and __cp_mbtowc are functions returning the
  right function pointer now.
- Create __WCTOMB/__MBTOWC macros utilizing per-reent locale and replace
  calls to __wctomb/__mbtowc with calls to __WCTOMB/__MBTOWC.
- Drop global __wctomb/__mbtowc vars.
- Utilize aforementioned changes in Cygwin to get rid of charset in other,
  calling functions and simplify the code.
- In Cygwin restrict global cygheap locale info to the job performed
  by internal_setlocale.  Use UTF-8 instead of ASCII on the fly in
  internal conversion functions.
- In Cygwin dll_entry, make sure to initialize a TLS area with a NULL
  _REENT->_locale pointer.  Add comment to explain why.

Signed-off by: Corinna Vinschen <corinna@vinschen.de>

winsup/cygwin/init.cc

@@ -83,6 +83,15 @@ dll_entry (HANDLE h, DWORD reason, void *static_load)
       cygwin_hmodule = (HMODULE) h;
       dynamically_loaded = (static_load == NULL);
 
+      /* Starting with adding the POSIX-1.2008 per-thread locale functionality,
+	 we need an initalized _REENT area even for the functions called from
+	 dll_crt0_0.  In fact, we only need the _REENT->_locale pointer
+	 initialized to NULL, so subsequent calls to locale-specific functions
+	 will always fall back to __global_locale, rather then crash due to
+	 _REENT->_locale having an arbitrary value. */
+      (void) alloca (CYGTLS_PADSIZE);
+      _REENT->_locale = NULL;
+
       dll_crt0_0 ();
       _my_oldfunc = TlsAlloc ();
       dll_finished_loading = true;