Hello,

Notmuch contains a bit of logic for “normalising” the database path: it can
be relative to $HOME and fallsback to $MAILDIR or $HOME/mail. However, this
logic is implemented in `notmuch_config_open` and is not available through
bindings. So if a third-party tool wants to discover the database, it will
either do this incorrectly ([alot]) or will have to roll its own
implementation ([afew]). Neither option is good.

[alot]: https://github.com/pazz/alot/issues/1396
[afew]: https://github.com/afewmail/afew/pull/233

Rather than reimplementing it in bindings, I see two possible solutions:

1. Move the logic to `notmuch_database_open` itself.
2. Extract into a separate C function, use it in `notmuch_config_open`, and
expose through bindings.

What do you think?

Cheers,
Kirill