Do not treat the command line or environment like paths

* dcrt0.cc (dll_crt0_1), environ.cc (environ_init, getwinenveq,
	build_env), strfuncs.cc (sys_wcstombs, sys_wcstombs_alloc),
	wchar.c (sys_wcstombs, sys_wcstombs_alloc): avoid mis-conversions
	of text that does not, actually, refer to a path or file name

Detailed explanation:

Our WCS -> UTF conversion handles the private Unicode page specially
to allow for otherwise invalid file names. However, this handling makes
no sense for command-lines, nor environment variables, which we would
rather convert verbatim.

As a stop-gap solution, let's just introduce a version of the
sys_wcstombs() function that specifically excludes that file name
conversion magic.

The proper solution is to change sys_wcstombs() to assume that it is not
a path that wants to be converted, and introduce sys_wcstombs_path()
that does, but that is a bigger task which we leave for another patch.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

winsup/cygwin/wchar.h

@@ -52,8 +52,12 @@ extern char *__locale_charset ();
 #ifdef __cplusplus
 size_t __reg3 sys_wcstombs (char *dst, size_t len, const wchar_t * src,
 			       size_t nwc = (size_t) -1);
+size_t __reg3 sys_wcstombs_no_path (char *dst, size_t len,
+			       const wchar_t * src, size_t nwc = (size_t) -1);
 size_t __reg3 sys_wcstombs_alloc (char **, int, const wchar_t *,
 				     size_t = (size_t) -1);
+size_t __reg3 sys_wcstombs_alloc_no_path (char **, int, const wchar_t *,
+				     size_t = (size_t) -1);
 
 size_t __reg3 sys_cp_mbstowcs (mbtowc_p, const char *, wchar_t *, size_t,
 				  const char *, size_t = (size_t) -1);