CONTRIB: move 51Degrees to addons/51degrees

Now it's much cleaner, both 51d.c and the dummy library live together and
are easier to spot and maintain. The build howto probably ought to be moved
there as well. Makefile, docs and MAINTAINERS were updated, as well as
the github CI's build matrix, travis CI's, and coverity checks.
diff --git a/doc/51Degrees-device-detection.txt b/doc/51Degrees-device-detection.txt
index f0349ab..8c69bb1 100644
--- a/doc/51Degrees-device-detection.txt
+++ b/doc/51Degrees-device-detection.txt
@@ -54,14 +54,14 @@
 key at https://51degrees.com/products/store/on-premise-device-detection.
 
 For HAProxy developers who need to verify that their changes didn't affect the
-51Degrees implementation, a dummy library if provided in the contrib/51d
-directory. This does not function, but implements the API such that the
-51Degrees module can be used (but not return any meaningful information). To
-test either Pattern or Hash Trie, build with:
+51Degrees implementation, a dummy library is provided in the
+"addons/51degrees/dummy" directory. This does not function, but implements the
+API such that the 51Degrees module can be used (but not return any meaningful
+information). To test either Pattern or Hash Trie, build with:
 
-    $ make TARGET=<target> USE_51DEGREES=1 51DEGREES_SRC=contrib/51d/src/pattern
+    $ make TARGET=<target> USE_51DEGREES=1 51DEGREES_SRC=addons/51degrees/dummy/pattern
 or
-    $ make TARGET=<target> USE_51DEGREES=1 51DEGREES_SRC=contrib/51d/src/trie
+    $ make TARGET=<target> USE_51DEGREES=1 51DEGREES_SRC=addons/51degrees/dummy/trie
 
 respectively.