LAPD: Add support for RTS based polling and T200

The T200 timer is started when the current frame is polled at
PH-READY-TO-SEND event.

A flag is used to enable this feature. The user of LAPD core must track
frame numbers to check the timeout condition. Then it must call the
external timeout function.

Related: OS#4074
Change-Id: Ib961b5a44911b99b0487641533301749c0286995
diff --git a/TODO-RELEASE b/TODO-RELEASE
index b67161d..1d56d45 100644
--- a/TODO-RELEASE
+++ b/TODO-RELEASE
@@ -9,3 +9,4 @@
 #library	what			description / commit summary line
 core      ADD       osmo_sock_multiaddr_{add,del}_local_addr()
 core      ADD       gsmtap_inst_fd2() core, DEPRECATE gsmtap_inst_fd()
+isdn		ABI change		add states and flags for external T200 handling