CONTRIB: move src/da.c and contrib/deviceatlas to addons/deviceatlas

Both the source file and the dummy library are now at the same place.
Maybe the build howto could be moved there as well to make things even
cleaner.

The Makefile, MAINTAINERS, doc, github build matrix, coverity checks
and travis CI's build were updated.
diff --git a/MAINTAINERS b/MAINTAINERS
index 144cc42..ff09b08 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -28,7 +28,7 @@
 
 DeviceAtlas device identification
 Maintainer: David Carlier <dcarlier@deviceatlas.com>
-Files: src/da.c, contrib/deviceatlas, doc/DeviceAtlas-device-detection.txt
+Files: addons/deviceatlas, doc/DeviceAtlas-device-detection.txt
 
 DNS and Resolvers
 Maintainer: Emeric Brun <ebrun@haproxy.com>