www/linux-firefox user-agent string

[Andrew Pantyukhin]

I maintain linux-firefox and linux-seamonkey ports.

Naturally linux browsers have "Linux" by default in
their user-agent strings. We can alter the default behaviour
and put FreeBSD instead of Linux there, or even something
more elaborate like "FreeBSD Linux-compat". We can
make it optional via WITHOUT_.

I'd like to know what you guys think about it. HTTP 1.0/1.1
RFCs state:

[User-Agent request-header field] is for statistical purposes,
the tracing of protocol violations, and automated recognition
of user agents for the sake of tailoring responses to avoid
particular user agent limitations.

I think that displaying FreeBSD is more to the spirit of this,
concerning that Linux-compat isn't a full blown emulation,
but just a compatibility layer.