Add g_get_environ(): portable access to 'environ'

Return a copy of 'environ' on platforms where that is possible, or do
something else on other platforms.

glib/gutils.h

@@ -258,6 +258,7 @@ gboolean              g_setenv             (const gchar *variable,
 					    gboolean     overwrite);
 void                  g_unsetenv           (const gchar *variable);
 gchar**               g_listenv            (void);
+gchar**               g_get_environ        (void);
 
 /* private */
 const gchar*	     _g_getenv_nomalloc	   (const gchar	*variable,