LAPDm: Add support for RTS based polling

The lower layer must set the 'POLLING_ONLY' flag and provide frame
number when polling a frame. If T200 is pending, it is started with a
timeout frame number in advance to given frame number.

The lower layer must call lapdm_t200_fn() after a frame has been
received or if a frame has not been received. Also it must be called
after a TCH frame has been received. LAPDm uses this to check the T200
timeout condition.

A new function is used to set the frame number based timeout values.

Related: OS#4074
Change-Id: I6ebe83f829d7751ea9de1d90eb478c7a628db64c
diff --git a/TODO-RELEASE b/TODO-RELEASE
index 1d56d45..226450b 100644
--- a/TODO-RELEASE
+++ b/TODO-RELEASE
@@ -10,3 +10,4 @@
 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
+gsm		ABI change		add T200 timer states to lapdm_datalink