Optional parameter U= to define a user agent in http forms

Allows the user to define the user agent used in http-{post|get}-form by adding the optional argument U (e.g.
``:U=Mozilla/5.0 Hydra``) to the command. ``^USER^`` and ``^PASS^`` are not replaced.
The user-agent in hydra-http-proxy.c is still hard coded. 

Hope you can use it.