Enhancement: Make `init` open only one connection for the SFTP backend

The `init` command using the SFTP backend used to connect twice to the
repository. This could be inconvenient if the user must enter a password,
or cause `init` to fail if the server does not correctly close the first SFTP
connection.

This has now been fixed by reusing the first/initial SFTP connection opened.

https://github.com/restic/restic/issues/2152
https://github.com/restic/restic/pull/3882
