BUG/MINOR: http-rules: Fix ACLs parsing for http deny rules

The parsing of http deny rules with no argument or only the deny_status argument
is buggy if followed by an ACLs expression (starting with "if" or "unless"
keyword). Instead of using the proxy errorfiles, a dummy error is used. To fix
the bug, the parsing function must also check for "if" or "unless" keyword in
such cases.

This patch should fix the issue #720. No backport is needed.
diff --git a/reg-tests/http-errorfiles/http_errors.vtc b/reg-tests/http-errorfiles/http_errors.vtc
index 7d9f18c..37e08cc 100644
--- a/reg-tests/http-errorfiles/http_errors.vtc
+++ b/reg-tests/http-errorfiles/http_errors.vtc
@@ -34,7 +34,7 @@
     frontend fe1
         bind "fd@${fe1}"
         http-request deny deny_status 400 if { path /400 }
-        http-request deny deny_status 403 if { path /403 }
+        http-request deny if { path /403 }
         http-request deny deny_status 404 if { path /404 }
         http-request deny deny_status 500 if { path /500 }
 
@@ -43,7 +43,7 @@
         errorfiles errors-1
         errorfile 500  ${testdir}/errors/500.http
         http-request deny deny_status 400 if { path /400 }
-        http-request deny deny_status 403 if { path /403 }
+        http-request deny if { path /403 }
         http-request deny deny_status 404 if { path /404 }
         http-request deny deny_status 500 if { path /500 }
 
@@ -53,7 +53,7 @@
         errorfiles errors-1 500
         errorfiles errors-3 400
         http-request deny deny_status 400 if { path /400 }
-        http-request deny deny_status 403 if { path /403 }
+        http-request deny if { path /403 }
         http-request deny deny_status 404 if { path /404 }
         http-request deny deny_status 500 if { path /500 }
 } -start