MEDIUM: mworker: depreciate the 'program' section

The program section is unreliable and should not be used, more reliable
alternatives exist outside HAProxy. Let's depreciate the section so we
could remove it completely in 3.3.

doc/configuration.txt

@@ -56,7 +56,7 @@ Summary
 3.4.      Userlists
 3.5.      Peers
 3.6.      Mailers
-3.7.      Programs
+3.7.      Programs (deprecated)
 3.8.      HTTP-errors
 3.9.      Rings
 3.10.     Log forwarding
@@ -4763,8 +4763,14 @@ timeout mail <time>
         timeout mail 20s
         mailer smtp1 192.168.0.1:587
 
-3.7. Programs
--------------
+3.7. Programs (deprecated)
+--------------------------
+
+This section is deprecated and should disappear with HAProxy 3.3. The section
+could be replaced easily by separated process managers. Systemd unit files or
+sysvinit scripts could replace this section as they are more reliable. In docker
+environments, some alternatives can also be found such as s6 or supervisord.
+
 In master-worker mode, it is possible to launch external binaries with the
 master, these processes are called programs. These programs are launched and
 managed the same way as the workers.

src/mworker-prog.c

@@ -194,6 +194,10 @@ int cfg_parse_program(const char *file, int linenum, char **args, int kwm)
 			goto error;
 		}
 
+		ha_warning("parsing [%s:%d]: The '%s' section is deprecated and will eventually be removed, please consider "
+		           "using a process manager instead, such as sysvinit, systemd, supervisord or s6\n",
+		           file, linenum, args[0]);
+
 		LIST_APPEND(&proc_list, &ext_child->list);
 
 	} else if (strcmp(args[0], "command") == 0) {