REGTESTS: enable -dW on almost all tests to fail on warnings

Now that warnings were almost all removed, let's enable zero-warning
via -dW. All tests were adjusted, but two:

  - mcli/mcli_start_progs.vtc:
      the programs section currently cannot be silenced

  - stats/stats-file.vtc:
      the warning comes from the stats file itself on comment lines.

All other ones are now OK.

reg-tests/mcli/mcli_debug_dev.vtc

@@ -19,7 +19,7 @@ syslog S1 -level info {
 } -start
 
 
-haproxy h1 -W -S -conf {
+haproxy h1 -W -S -arg '-dW' -conf {
     global
 	log ${S1_addr}:${S1_port} local0 info
 

reg-tests/mcli/mcli_show_info.vtc

@@ -6,7 +6,7 @@ feature ignore_unknown_macro
 server s1 {
 } -start
 
-haproxy h1 -W -S -conf {
+haproxy h1 -W -S -arg '-dW' -conf {
     defaults
         mode http
         timeout connect "${HAPROXY_TEST_TIMEOUT-5s}"