add mDNS lookup method to libosmo-mslookup

Add the first actually useful lookup method to the mslookup library: multicast
DNS.

The server side is added in a subsequent commit, when the mslookup server is
implemented for the osmo-hlr program.

Use custom DNS encoding instead of libc-ares (which we use in OsmoSGSN
already), because libc-ares is only a DNS client implementation and we will
need both client and server.

Related: OS#4237
Patch-by: osmith, nhofmeyr
Change-Id: I03a0ffa1d4dc1b24ac78a5ad0975bca90a49c728
diff --git a/include/osmocom/mslookup/Makefile.am b/include/osmocom/mslookup/Makefile.am
new file mode 100644
index 0000000..72ccd94
--- /dev/null
+++ b/include/osmocom/mslookup/Makefile.am
@@ -0,0 +1,6 @@
+# most headers here are installed, see /include/Makefile.am
+
+noinst_HEADERS = \
+	mdns_msg.h \
+	mdns_rfc.h \
+	$(NULL)