Move the core environment parsing of environ_init into a new win32env_to_cygenv function.

win32env_to_cygwenv handles converting wchar to char and some other
minor taks.  Optionally it handles converting any paths in variables to
posix paths.

This will be useful for implementing /proc/<pid>/environ

winsup/cygwin/environ.h

@@ -45,4 +45,6 @@ extern "C" char __stdcall **cur_environ ();
 char ** __reg3 build_env (const char * const *envp, PWCHAR &envblock,
 			  int &envc, bool need_envblock, HANDLE new_token);
 
+char ** __reg2 win32env_to_cygenv (PWCHAR rawenv, bool posify);
+
 #define ENV_CVT -1