BUG/MINOR: haproxy: Fix option to disable the fast-forward

The option was renamed to only permit to disable the fast-forward. First
there is no reason to enable it because it is the default behavior. Then it
introduced a bug because there is no way to be sure the command line has
precedence over the configuration this way. So, the option is now named
"tune.disable-fast-forward" and does not support any argument. And of
course, the commande line option "-dF" has now precedence over the
configuration.

No backport needed.
diff --git a/doc/configuration.txt b/doc/configuration.txt
index 3b1e29a..b13b97c 100644
--- a/doc/configuration.txt
+++ b/doc/configuration.txt
@@ -1123,7 +1123,7 @@
    - tune.buffers.reserve
    - tune.bufsize
    - tune.comp.maxlevel
-   - tune.fast-forward
+   - tune.disable-fast-forward
    - tune.fd.edge-triggered
    - tune.h2.header-table-size
    - tune.h2.initial-window-size
@@ -2832,23 +2832,22 @@
   Each session using compression initializes the compression algorithm with
   this value. The default value is 1.
 
+tune.disable-fast-forward [ EXPERIMENTAL ]
+  Disables the data fast-forwarding. It is a mechanism to optimize the data
+  forwarding by passing data directly from a side to the other one without
+  waking the stream up. Thanks to this directive, it is possible to disable
+  this optimization. Note it also disable any kernel tcp splicing. This command
+  is not meant for regular use, it will generally only be suggested by
+  developers along complex debugging sessions. For this reason it is internally
+  marked as experimental, meaning that "expose-experimental-directives" must
+  appear on a line before this directive.
+
 tune.fail-alloc
   If compiled with DEBUG_FAIL_ALLOC or started with "-dMfail", gives the
   percentage of chances an allocation attempt fails. Must be between 0 (no
   failure) and 100 (no success). This is useful to debug and make sure memory
   failures are handled gracefully.
 
-tune.fast-forward { on | off } [ EXPERIMENTAL ]
-
-  Enabled ('on') or disables ('off') the data fast-forwarding. It is a
-  mechanism to optimize the data forwarding by passing data directly from a
-  side to the other one without waking the stream up. Thanks to this directive,
-  it is possible to disable this optimization. Note it also disable any kernel
-  tcp splicing. This command is not meant for regular use, it will generally
-  only be suggested by developers along complex debugging sessions. For this
-  reason it is internally marked as experimental, meaning that
-  "expose-experimental-directives" must appear on a line before this directive.
-
 tune.fd.edge-triggered { on | off }  [ EXPERIMENTAL ]
   Enables ('on') or disables ('off') the edge-triggered polling mode for FDs
   that support it. This is currently only support with epoll. It may noticeably