Add example osmo-mgw configuration file for Abis/E1

In this example, we are using the first span (0) of the first DAHDI card
and use it as 'trunk 1' in the MGW.

Change-Id: I0a97da5163a94379b327403b1258696855836bad

contrib/osmo-mgw.spec.in

@@ -113,6 +113,7 @@ make %{?_smp_mflags} check || (find . -name testsuite.log -exec cat {} +)
 %dir %{_docdir}/%{name}/examples
 %dir %{_docdir}/%{name}/examples/osmo-mgw
 %{_docdir}/%{name}/examples/osmo-mgw/osmo-mgw.cfg
+%{_docdir}/%{name}/examples/osmo-mgw/osmo-mgw-abis_e1.cfg
 %{_bindir}/osmo-mgw
 %{_unitdir}/osmo-mgw.service
 %dir %{_sysconfdir}/osmocom

debian/osmo-mgw.install

@@ -2,3 +2,4 @@ etc/osmocom/osmo-mgw.cfg
 lib/systemd/system/osmo-mgw.service
 usr/bin/osmo-mgw
 usr/share/doc/osmo-mgw/examples/osmo-mgw/osmo-mgw.cfg
+usr/share/doc/osmo-mgw/examples/osmo-mgw/osmo-mgw-abis_e1.cfg

doc/examples/osmo-mgw/osmo-mgw-abis_e1.cfg

@@ -0,0 +1,25 @@
+!
+! MGCP configuration example
+!
+e1_input
+ e1_line 0 driver dahdi
+ e1_line 0 port 0
+mgcp
+  bind ip 127.0.0.1
+  rtp port-range 4002 16000
+  rtp bind-ip 127.0.0.1
+  rtp ip-probing
+  rtp ip-tos 184
+  bind port 2427
+  sdp audio payload number 98
+  sdp audio payload name GSM
+  number endpoints 31
+  loop 0
+  force-realloc 1
+  rtcp-omit
+  rtp-patch ssrc
+  rtp-patch timestamp
+  trunk 1
+   rtp keep-alive once
+   no rtp keep-alive
+   line 0