MINOR: haproxy: Add an command option to disable data fast-forward

The -dF option can now be used to disable data fast-forward. It does the
same than the global option "tune.fast-forward off". Some reg-tests may rely
on this optim. To detect the feature and skip such script, the following
vtest command must be used:

  feature cmd "$HAPROXY_PROGRAM -cc '!(globa.tune & GTUNE_NO_FAST_FWD)'"

doc/management.txt

@@ -217,6 +217,13 @@ list of options is :
     suspicious configuration statements. This will never prevent startup even in
     "zero-warning" mode nor change the exit status code.
 
+  -dF : disable data fast-forward. 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.
+
   -dG : disable use of getaddrinfo() to resolve host names into addresses. It
     can be used when suspecting that getaddrinfo() doesn't work as expected.
     This option was made available because many bogus implementations of

reg-tests/http-messaging/http_splicing.vtc

@@ -4,6 +4,7 @@ varnishtest "A test to validate h1 splicing support"
 
 feature cmd "$HAPROXY_PROGRAM -cc 'version_atleast(2.6-dev0)'"
 feature cmd "$HAPROXY_PROGRAM -cc 'feature(LINUX_SPLICE)'"
+feature cmd "$HAPROXY_PROGRAM -cc '!(globa.tune & GTUNE_NO_FAST_FWD)'"
 feature ignore_unknown_macro
 
 #REGTEST_TYPE=slow

src/haproxy.c

@@ -622,6 +622,7 @@ static void usage(char *name)
 		"        -dV disables SSL verify on servers side\n"
 		"        -dW fails if any warning is emitted\n"
 		"        -dD diagnostic mode : warn about suspicious configuration statements\n"
+		"        -dF disable fast-forward\n"
 		"        -sf/-st [pid ]* finishes/terminates old pids.\n"
 		"        -x <unix_socket> get listening sockets from a unix socket\n"
 		"        -S <bind>[,<bind options>...] new master CLI\n"
@@ -1647,6 +1648,8 @@ static void init_args(int argc, char **argv)
 			else if (*flag == 'd' && flag[1] == 'R')
 				global.tune.options &= ~GTUNE_USE_REUSEPORT;
 #endif
+			else if (*flag == 'd' && flag[1] == 'F')
+				global.tune.options |= GTUNE_NO_FAST_FWD;
 			else if (*flag == 'd' && flag[1] == 'V')
 				global.ssl_server_verify = SSL_SERVER_VERIFY_NONE;
 			else if (*flag == 'V')