REORG: include: move backend.h to haproxy/backend{,-t}.h
The files remained mostly unchanged since they were OK. However, half of
the users didn't need to include them, and about as many actually needed
to have it and used to find functions like srv_currently_usable() through
a long chain that broke when moving the file.
diff --git a/src/backend.c b/src/backend.c
index 18a5697..6e6280a 100644
--- a/src/backend.c
+++ b/src/backend.c
@@ -20,6 +20,7 @@
#include <sys/types.h>
#include <haproxy/acl.h>
+#include <haproxy/backend.h>
#include <haproxy/api.h>
#include <haproxy/channel.h>
#include <haproxy/check.h>
@@ -49,7 +50,6 @@
#include <haproxy/global.h>
#include <haproxy/arg.h>
-#include <proto/backend.h>
#include <haproxy/protocol.h>
#include <haproxy/proto_tcp.h>
#include <proto/queue.h>