cli: move config open/close to main() from subcommands

This allows specifying config file as a top level argument to notmuch,
and generally makes it possible to override config file options in
main(), without having to touch the subcommands.

If the config file does not exist, one will be created for the notmuch
main command and setup and help subcommands. Help is special in this
regard; the config is created just to avoid errors about missing
config, but it will not be saved.

This also makes notmuch config the talloc context for subcommands.