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

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.