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.

CCBUG: 271720
REVIEW: 101337