The new test is in T055-path-config because it uses the helper
function split_config, and because it seems easier to put the
database path related tests in one place.
This changes some error reporting, either intentionally by reporting
the highest level missing directory, or by side effect from looking in
XDG locations when given null database location.
This adds new state variable for the mail root, and uses it most
places db_path was used. The notable exception is dumps from
backups. The latter will be dealt with properly in a future commit.
The main functionality will be tested when notmuch-new is converted to
support split configuration. Here only the somewhat odd case of split
mail root which is actually symlinked to the database path is tested.
Introduce a new configuration value for the mail root, and use it to
locate mail messages in preference to the database.path (which
previously implied the mail messages were also in this location.
Initially only a subset of the CLI is tested in a split
configuration. Further changes will be needed for the remainder of the
CLI to work in split configurations.
