ipsec: put back the lock, because...

rc.newwanip, rc.newwanipv6, interface_configure() and interfaces_configure()
all call ipsec_configure_do(), which is no wonder why the code can be prone
to races.

src/etc/rc.newipsecdns

@@ -45,4 +45,6 @@ if (file_exists('/var/run/booting')) {
     return;
 }
 
+$ipseclck = lock('ipsecdns', LOCK_EX);
 ipsec_configure_do(true);
+unlock($ipseclck);