Add default filename and divelist font to prefs structure

.. and add the usual logic to not save the default values.

This also simplifies the initial system-specific setup of both of these:
since we have defaults for all the preferences that get set up at
startup, we can just initialize those defaults to the system-specific
fonts then and there.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>