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/.gitignore b/.gitignore
index acfea84..eec364a 100644
--- a/.gitignore
+++ b/.gitignore
@@ -51,6 +51,8 @@
 tests/db/db_test
 tests/hlr_vty_test.db*
 tests/db_upgrade/*.dump
+tests/mslookup/mdns_test
+tests/mslookup/mslookup_client_mdns_test
 tests/mslookup/mslookup_client_test
 tests/mslookup/mslookup_test