Enhancement: Don't terminate ssh on SIGINT

We've reworked the code which runs the `ssh` login for the sftp backend so that
it can prompt for a password (if needed) but does not exit when the user
presses CTRL+C (SIGINT) e.g. during backup. This allows restic to properly shut
down when it receives SIGINT and remove the lock file from the repo, afterwards
exiting the `ssh` process.

https://github.com/restic/restic/pull/1564
https://github.com/restic/restic/pull/1588
