MEDIUM: acl/pattern: switch rdp_cookie functions stack up-down

MEDIUM: acl/pattern: switch rdp_cookie functions stack up-down

Previously, both pattern, backend and persist_rdp_cookie would build fake
ACL expressions to fetch an RDP cookie by calling acl_fetch_rdp_cookie().

Now we switch roles. The RDP cookie fetch function is provided as a sample
fetch function that all others rely on, including ACL. The code is exactly
the same, only the args handling moved from expr->args to args. The code
was moved to proto_tcp.c, but probably that a dedicated file would be more
suited to content handling.