updown: Explicitly pass caller PATH to updown script

When invoking /bin/sh, its default PATH is used. On some systems, that does
not include the PATH where the ipsec script is installed, as charon is invoked
with a custom PATH. Explicitly setting the PATH of charon should fix this
case, properly invoking the (default) updown script.

Fixes #745.

src/libcharon/plugins/updown/updown_listener.c

@@ -243,6 +243,7 @@ static void invoke_once(private_updown_listener_t *this, ike_sa_t *ike_sa,
 	me = ike_sa->get_my_host(ike_sa);
 	other = ike_sa->get_other_host(ike_sa);
 
+	push_env(envp, countof(envp), "PATH=%s", getenv("PATH"));
 	push_env(envp, countof(envp), "PLUTO_VERSION=1.1");
 	is_host = my_ts->is_host(my_ts, me);
 	if (is_host)