REORG: include: move channel.h to haproxy/channel{,-t}.h

The files were moved with no change. The callers were cleaned up a bit
and a few of them had channel.h removed since not needed.
diff --git a/src/stats.c b/src/stats.c
index a462056..5737215 100644
--- a/src/stats.c
+++ b/src/stats.c
@@ -27,6 +27,7 @@
 #include <haproxy/applet-t.h>
 #include <haproxy/api.h>
 #include <common/cfgparse.h>
+#include <haproxy/channel.h>
 #include <haproxy/check.h>
 #include <haproxy/cli.h>
 #include <haproxy/compression.h>
@@ -55,7 +56,6 @@
 #include <haproxy/base64.h>
 
 #include <proto/backend.h>
-#include <proto/channel.h>
 #include <haproxy/fd.h>
 #include <haproxy/freq_ctr.h>
 #include <proto/log.h>