MAJOR: http: Deprecate and ignore the option "http-use-htx"

From this commit, the legacy HTTP mode is now definitely disabled. It is the
first commit of a long series to remove the legacy HTTP code. Now, all HTTP
processing is done using the HTX internal representation. Since the version 2.0,
It is the default mode. So now, it is no more possible to disable the HTX to
fallback on the legacy HTTP mode. If you still use "[no] option http-use-htx", a
warning will be emitted during HAProxy startup. Note the passthough multiplexer
is now only usable for TCP proxies.
diff --git a/src/cfgparse-listen.c b/src/cfgparse-listen.c
index 9c98a23..b3a8c8d 100644
--- a/src/cfgparse-listen.c
+++ b/src/cfgparse-listen.c
@@ -2152,6 +2152,15 @@
 					goto out;
 				}
 
+				/* "[no] option http-use-htx" is deprecated */
+				if (!strcmp(cfg_opts2[optnum].name, "http-use-htx")) {
+					ha_warning("parsing [%s:%d]: option '%s' is deprecated and ignored."
+						   " The HTX mode is now the only supported mode.\n",
+						   file, linenum, cfg_opts2[optnum].name);
+					err_code |= ERR_WARN;
+					goto out;
+				}
+
 				curproxy->no_options2 &= ~cfg_opts2[optnum].val;
 				curproxy->options2    &= ~cfg_opts2[optnum].val;
 
@@ -2704,7 +2713,7 @@
 			int cur_arg;
 
 			/* insert x-forwarded-for field, but not for the IP address listed as an except.
-			 * set default options (ie: bitfield, header name, etc) 
+			 * set default options (ie: bitfield, header name, etc)
 			 */
 
 			curproxy->options |= PR_O_FWDFOR | PR_O_FF_ALWAYS;
@@ -3674,7 +3683,7 @@
 			goto out;
 		}
 
-		/* we must first clear any optional default setting */	
+		/* we must first clear any optional default setting */
 		curproxy->conn_src.opts &= ~CO_SRC_TPROXY_MASK;
 		free(curproxy->conn_src.iface_name);
 		curproxy->conn_src.iface_name = NULL;