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/channel.c b/src/channel.c
index ae5c981..a3b7b9e 100644
--- a/src/channel.c
+++ b/src/channel.c
@@ -17,8 +17,7 @@
 
 #include <haproxy/api.h>
 #include <haproxy/buf.h>
-
-#include <proto/channel.h>
+#include <haproxy/channel.h>
 
 
 /* Schedule up to <bytes> more bytes to be forwarded via the channel without