lapd: Always print context information when logging

Historically, OpenBSC has primarily been used with setups that have
a single E1 based BTS connected. This meant that an error message on
the E1 LAPD implicitly has to be related to that single BTS.

However, in more comprehensive setups, there may be many BTSs on many
E1 lines with many signaling slots.  At this point, it's important to
know which line/timeslot/tei/sapi a given log message relates to.

This patch introduces related log context.

Change-Id: Ib81a749ae24013b17caaf5fd64ccd9acbbc3ce08
Requires: libosmocore.git Change-Id Ie6742843fff809edffcac24c4dce4edf66bc71be
Related: OS#1938
diff --git a/TODO-RELEASE b/TODO-RELEASE
index e49df7a..85b16a7 100644
--- a/TODO-RELEASE
+++ b/TODO-RELEASE
@@ -9,3 +9,4 @@
 #library	what		description / commit summary line
 libosmo-abis	API change	major: add parameter to struct e1inp_line
 libosmo-trau	API change	add new function osmo_rtp_socket_set_dscp()
+libosmo-abis	API change	major: add parameter to struct lapd_instance