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

configure.ac

@@ -174,10 +174,12 @@ AC_OUTPUT(
 	doc/examples/Makefile
 	src/Makefile
 	src/gsupclient/Makefile
+	src/mslookup/Makefile
 	include/Makefile
 	include/osmocom/Makefile
 	include/osmocom/hlr/Makefile
 	libosmo-gsup-client.pc
+	libosmo-mslookup.pc
 	sql/Makefile
 	doc/manuals/Makefile
 	contrib/Makefile
@@ -188,4 +190,5 @@ AC_OUTPUT(
 	tests/gsup_server/Makefile
 	tests/db/Makefile
 	tests/db_upgrade/Makefile
+	tests/mslookup/Makefile
 	)