This is part of a fix for reproducible builds where usrmerge may or may
not be in effect on the build system. We need to be able to pass the
BASH envvar to autotools so its able to correctly determine the correct
path for bash. The lines removed here were added to fix DEBBUG-346303
but should not be required on a clean build environment, where BASH
remains unset unless the current shell is actually BASH, and when it is,
should return the correct path to the bash executable and not /bin/sh.

That way GitLab is able to convert both internal bug ids and Debian bug
tracker numbers to hyperlinks in the rendered version.

Add new restic handler

See merge request liberate/backupninja!48

Without it, the single-quoted paths on the command-line are interperted
literally, leading restic find all include paths invalid...

Make this consistent with the borg handler. Allows users to use paths
that include spaces in them and prevents shell globbing when the restic
command is called. According to the restic docs, if glob paths are
needed, they should be read from a file (--files-from).

Making this consistent with the borg handler. Users should think about
what they need to be backup. The example file is there to provide a
baseline.

Add backup/init config directive (defaults to yes) to control whether
the repository should be initialized if it doesn't exist. Since we need
to attempt to connect to the repository to verify if init is needed,
remove the testconnect parameter so that the handler just tests the
connection every time it runs.

Replace convoluted code to test SFTP and REST backends with a simple
restic snapshots command. Much simpler and supports all backends...

As per the restic docs, check should usually be run after prune, which
makes sense as we want to verify the consistency of the repository after
making changes.

The command rebuild-index should only be necessary as part of an attempt
to repair a malfunctioning repository. I could not find anywhere in the
documentation recommending to run this command at regular intervals.