create libosmoisdn sub-library

There are some parts of libosmogsm which are not really GSM specific,
but rather ISDN bits that were inherited by GSM.  This includes the
I.460 multiplex as well as the core LAPD protocol.

Let's move those bits to its own libosmoisdn library, before we add
more ISDN specific bits to the wrong place.

Backwards-compatibility is created by making libosmogsm depend on
libosmoisdn, and by providing wrapper include files for source
compatibility.

Change-Id: Ib1a6c762322fd5047be3188b1df22408ef06aa50
---
M .gitignore
M Doxyfile.codec.in
M Doxyfile.coding.in
M Doxyfile.core.in
M Doxyfile.ctrl.in
M Doxyfile.gb.in
M Doxyfile.gsm.in
A Doxyfile.isdn.in
M Doxyfile.vty.in
M Makefile.am
M configure.ac
M contrib/libosmocore.spec.in
M debian/control
M debian/copyright
M debian/libosmocore.dirs
A debian/libosmoisdn-doc.doc-base
A debian/libosmoisdn-doc.install
A debian/libosmoisdn0.install
M debian/rules
M include/Makefile.am
M include/osmocom/gsm/i460_mux.h
M include/osmocom/gsm/lapd_core.h
A include/osmocom/isdn/i460_mux.h
A include/osmocom/isdn/lapd_core.h
M libosmogsm.pc.in
A libosmoisdn.pc.in
M src/gsm/Makefile.am
A src/isdn/Makefile.am
R src/isdn/i460_mux.c
R src/isdn/lapd_core.c
A src/isdn/libosmoisdn.map
M tests/Makefile.am
M tests/i460_mux/i460_mux_test.c
33 files changed, 2,206 insertions(+), 311 deletions(-)