add libosmo-mslookup abstract client

mslookup is a key concept in Distributed GSM, which allows querying the current
location of a subscriber in a number of cooperating but independent core
network sites, by arbitrary service names and by MSISDN/IMSI.

Add the abstract mslookup client library. An actual lookup method (besides
mslookup_client_fake.c) is added in a subsequent patch.

For a detailed overview of this and upcoming patches, please see the elaborate
comment at the top of mslookup.c.

Add as separate library, libosmo-mslookup, to allow adding D-GSM capability to
arbitrary client programs.

osmo-hlr will be the only mslookup server implementation, added in a subsequent
patch.

osmo-hlr itself will also use this library and act as an mslookup client, when
requesting the home HLR for locally unknown IMSIs.

Related: OS#4237
Patch-by: osmith, nhofmeyr
Change-Id: I83487ab8aad1611eb02e997dafbcb8344da13df1

debian/control

@@ -59,6 +59,28 @@ Description: Development headers of Osmocom GSUP client library
   .
   This package contains the development headers.
 
+Package: libosmo-mslookup0
+Section: libs
+Architecture: any
+Multi-Arch: same
+Depends: ${shlibs:Depends},
+         ${misc:Depends}
+Pre-Depends: ${misc:Pre-Depends}
+Description: Osmocom MS lookup library
+  This shared library contains routines for looking up mobile subscribers.
+
+Package: libosmo-mslookup-dev
+Architecture: any
+Multi-Arch: same
+Depends: ${misc:Depends},
+	 libosmo-mslookup0 (= ${binary:Version}),
+	 libosmocore-dev
+Pre-Depends: ${misc:Pre-Depends}
+Description: Development headers of Osmocom MS lookup library
+  This shared library contains routines for looking up mobile subscribers.
+  .
+  This package contains the development headers.
+
 Package: osmo-hlr-doc
 Architecture: all
 Section: doc