Revamp the installation instructions for FreeBSD

The documentation on how to have CouchDB installed and enabled as
a service on FreeBSD contains a number of outdated information.
This change is to bring it to up to date along the following
lines:

- The port/package has been renamed to `couchdb3`.
- The configuration options and location for the configuration
  files for the port have changed.
- Propagate the `service` command to work with the wrapper script
  instead of accessing the script itself directly.
- Mention the binary package path (installation via `pkg`) which
  has become quite a standard these days.
- Remove instructions on manually creating the `couchdb` user and
  group.  There is no need for those any more, it shall be handled
  by the port/package.
1 file changed