BUG/MINOR: server: reject enabled for dynamic server

Since their first implementation, dynamic servers are created into
maintenance state. This has been done purposely to avoid immediate
activation of a newly inserted server.

However, this principle is incompatible if "enabled" keyword is used on
"add server". The newly created instance will be unreacheable as proxy
load-balancing algorithm is not informed of its presence via
srv_lb_propagate(). The new server could be unblocked by toggling its
state with "disable server" / "enable server" commands, which will
trigger srv_lb_propagate() invocation.

To avoid this unexpected state, simply forbid "enabled" keyword for
dynamic servers. In the long-term, it could be possible to re authorize
it but at least this requires to call srv_lb_propagate() on dynamic
server creation.

This should fix github issue #2497.

This patch should not be backported as-is, to avoid breaking dynamic
servers API on stable versions. "enabled" should instead be ignored for
them. This will be implemented in a dedicated patch on top of 2.9.

doc/management.txt

@@ -1696,7 +1696,6 @@ add server <backend>/<server> [args]*
   - crt
   - disabled
   - downinter
-  - enabled
   - error-limit
   - fall
   - fastinter

src/server.c

@@ -2218,7 +2218,7 @@ static struct srv_kw_list srv_kws = { "ALL", { }, {
 	{ "backup",               srv_parse_backup,               0,  1,  1 }, /* Flag as backup server */
 	{ "cookie",               srv_parse_cookie,               1,  1,  0 }, /* Assign a cookie to the server */
 	{ "disabled",             srv_parse_disabled,             0,  1,  1 }, /* Start the server in 'disabled' state */
-	{ "enabled",              srv_parse_enabled,              0,  1,  1 }, /* Start the server in 'enabled' state */
+	{ "enabled",              srv_parse_enabled,              0,  1,  0 }, /* Start the server in 'enabled' state */
 	{ "error-limit",          srv_parse_error_limit,          1,  1,  1 }, /* Configure the consecutive count of check failures to consider a server on error */
 	{ "ws",                   srv_parse_ws,                   1,  1,  1 }, /* websocket protocol */
 	{ "id",                   srv_parse_id,                   1,  0,  1 }, /* set id# of server */