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/Makefile b/Makefile
index 3a295c7..a9e8b49 100644
--- a/Makefile
+++ b/Makefile
@@ -652,7 +652,7 @@
 51DEGREES_LIB = $(51DEGREES_SRC)
 OPTIONS_OBJS    += $(51DEGREES_LIB)/../cityhash/city.o
 OPTIONS_OBJS    += $(51DEGREES_LIB)/51Degrees.o
-OPTIONS_OBJS    += src/51d.o
+OPTIONS_OBJS    += addons/51degrees/51d.o
 OPTIONS_CFLAGS  += $(if $(51DEGREES_INC),-I$(51DEGREES_INC))
 ifeq ($(USE_THREAD),)
 OPTIONS_CFLAGS  += -DFIFTYONEDEGREES_NO_THREADING
@@ -1004,6 +1004,7 @@
 	$(Q)rm -f haproxy-$(VERSION) haproxy-$(VERSION)$(SUBVERS)$(EXTRAVERSION) nohup.out gmon.out
 	$(Q)rm -f {admin,dev,contrib}/*/*.[oas] {admin,dev,contrib}/*/*/*.[oas] {admin,dev,contrib}/*/*/*/*.[oas]
 	$(Q)rm -f addons/promex/*.[oas]
+	$(Q)rm -f addons/51degrees/*.[oas] addons/51degrees/dummy/*.[oas] addons/51degrees/dummy/*/*.[oas]
 	$(Q)rm -f admin/iprange/iprange admin/iprange/ip6range admin/halog/halog
 	$(Q)rm -f dev/flags/flags dev/poll/poll dev/tcploop/tcploop
 	$(Q)rm -f dev/hpack/decode dev/hpack/gen-enc dev/hpack/gen-rht