Changed the default user-agent string so that it is more aligned with other
browsers by:

- dropping the 'comptiable' and 'U' from the string.
- making the platform string (e.g. X11) mandatory.
- sending the OS and machine type by default, e.g. Linux i686.

Besides being good for privacy (less differentiation), this change should make
it less likely that a different page will be sent to KDE based browsers.


REVIEW: 101337