gutils: Add functions for working with environment arrays

When spawning a child process, it is not safe to call setenv() before
the fork() (because setenv() isn't thread-safe), but it's also not
safe to call it after the fork() (because it's not async-signal-safe).
So the only safe way to alter the environment for a child process from
a threaded program is to pass a fully-formed envp array to
exec*/g_spawn*/etc.

So, add g_environ_getenv(), g_environ_setenv(), and
g_environ_unsetenv(), which act like their namesakes, but work on
arbitrary arrays rather than working directly on the environment.

http://bugzilla.gnome.org/show_bug.cgi?id=659326

docs/reference/glib/glib-sections.txt

@@ -1665,6 +1665,9 @@ g_set_application_name
 g_get_prgname
 g_set_prgname
 g_get_environ
+g_environ_getenv
+g_environ_setenv
+g_environ_unsetenv
 g_getenv
 g_setenv
 g_unsetenv