Add URI lookkup methods to Sqitch.pm.

These will be used both by commands that have database options and args passed
to them, as well as by the engine when dealing with the default URI.
Assume default DB might be a config key.

In other words, it can be a URI or a key to look up in the "database" section
of the config.
Rename the db_uri config to database.

Because it can be a key or a URI. Requires the addition of the `database`
attribute to Engine, purely for configuration purposes.
Remove comment; adjust whitespace.
Always merge options into URI.

It's just more consistent, and therefore less likely to be confusing. xThe
options get highest priority, as has always been the case.
Add the database param and option to status.
Remove password from destination.

This is used for display, so it should not include the password.
Make sure no engine includes password in destination or meta_destination.
Pass URI params to connect().
Make db_uri read-only.
Rename the Postgres Sqitch metadata schema attribute "registry".

Applies to the configuration, too.
Rename the mysql "sqitch_db" attribute to "registry".
Suppress password in the MySQL meta destination.
Rename SQLite sqitch_db attribute to "registry".

Well, it was sqitch_db_uri, but I've made "registry" a file base name. Noticed
that the code to produce the registry database name was incorrectly assuming
that the db_uri database name would always have an extension, which is not
true. So fixed that, and allowed the registry to have its own extension, in
which case it won't keep the old extension.
Allow SQLite registry to be an absolute path.

And add `core.sqlite.registry` to the config docs.
Rename the Oracle sqitch_schema attribute to "registry".
Move registry attribute to Engine.

Only override it in subclasses where there is some variation in the treatment
of defaults. The SQLite default does not include the deprecated config, as it
is used in `registry_uri`. That can be removed when the deprecated configs are
removed. The Oracle default is `undef`, not "sqitch", so that will be a
permanent override.

Also changed `db_uri` to use `name()` instead of `sqitch->_engine` to fetch
the name to use as the "engine" part of the URI and to fetch config values.
Remove config_vars from engine subclasses.

They are now the same for *all* engines. Yay!
Update change info on config vars.
Replace the term "metadata" with "registry".

Includes renaming the "meta_destination" attribute to "reg_destination".