kbs2: Use KBS2_CONFIG_DIR as a default config dir value

[woodruffw]

This establishes the following precedence:

• --config-dir DIR
• KBS2_CONFIG_DIR
• The default, i.e. ~/.config/kbs2

This has the nice consequence of ensuring that custom subcommands use the right configuration directory when invoking kbs2 on their own without requiring them to do something ugly like:

$ kbs2 -c "${KBS2_CONFIG_DIR}" whatever -a -b -c

Hooks are not guaranteed to receive KBS2_CONFIG_DIR in their environment. This probably needs to be fixed and will require a more involved change (since a Config doesn't know the directory that produced it).