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/cygheap.cc

@@ -28,7 +28,7 @@
 
 static mini_cygheap NO_COPY cygheap_dummy =
 {
-  {__utf8_mbtowc, __utf8_wctomb}
+  {__utf8_mbtowc}
 };
 
 init_cygheap NO_COPY *cygheap = (init_cygheap *) &cygheap_dummy;
@@ -245,8 +245,6 @@ cygheap_init ()
 	cygheap->bucket_val[b] = sz[b & 1];
       /* Default locale settings. */
       cygheap->locale.mbtowc = __utf8_mbtowc;
-      cygheap->locale.wctomb = __utf8_wctomb;
-      strcpy (cygheap->locale.charset, "UTF-8");
       /* Set umask to a sane default. */
       cygheap->umask = 022;
       cygheap->rlim_core = RLIM_INFINITY;