sysupgrade: autodetect of the used stable branch in default.conf

[Robert Bagdan <kikadf.01%gmail.com@localhost>]

Hi,
I've added changes to the configure file of sysupgrade to detect the
used branch. With this 'sysupgrade auto' will upgrade to the latest
snapshot of the used branch by default. Changes are in the pkgsrc-wip
repo.

Regards,
kikadf