Introduce 'rvn configure' subcommand?

[kraileth]

One of the things that make ravenadm(8) really nice to work with is that you basically never have to edit its ini file yourself. The simple configure menu adds a lot to the acessibility for new users and to convenience for experienced users. Also consistency across tools of the same family is always nice.

Example for what the menu might look like:

----------------------------------- Repository -----------------------------------
   [A] Repository URL				http://www.ravenports.com/repository/${ABI}
   [B] Signature type				pubkey
   [C] Keyfile					/raven/etc/rvn/keys/ravenports.key
---------------------------- Subpackages and variants -----------------------------
   [D] Install manpage SPKGs by default		yes
   [E] Install i18n SPKGs by default		no
   ...
   [F] Preferred variants if available		none
-------------------------------------- Paths --------------------------------------
   [G] DB path					/var/db/rvn
   [H] Package cache path			/var/cache/rvn

So most are pretty obvious examples. F is something that e.g. desktop users may want to set. If unset, all that the package manager will ever install unless explicitly told otherwise is the :standard variant of packages. However if F is changed to "x11,loaded", then rvn will prefer :x11 over the default (for packages where that is available in addition to :standard of course) and also prefer :loaded over :standard if no :x11 exists.