lei q|up: support v2:/path/to/inboxdir destination

lei q|up: support v2:/path/to/inboxdir destination

This allows "lei-managed pseudo mailing lists" as described
by Konstantin.

Alternates use is optional and can be enables via --shared.

This doesn't manage or edit ~/.public-inbox/config; presumably
there'll need to be some tweaking of search parameters before
finalizing and making the inbox publicly accessible via HTTP/NNTP.

Link: https://public-inbox.org/meta/20210426164454.5zd5kgugfhfwfkpo@nitro.local/T/