Create bee template from mTEMPLATE.be0

Add option useroot=false to disable install_helper.sh from the package
to use chown and chmod u+s on fusermount3. This is so that the bee
script can be called without root (`unshare -r`)

Add `chmod u+s $D$BINDIR/fusermount3` because it is no longer done by
install_helper.sh.

Create be0 file from mTEMPLATE.be0.

The proxmox backup software is open source and it should be possible to
build it from sources. However, the build system is strongly tied to
Debian, so it would be a lot of work to port it or to set up some
container build system.

For the evaluation phase now, we can postpone this and install the
binaries from the Debian package instead.

This bee file installs these files from the Debian package:

- /usr/bin/proxmox-backup-client
- /usr/bin/pxar
- /usr/share/man/man1/proxmox-backup-client.1.gz
- /usr/share/man/man1/pxar.1.gz
- /usr/share/bash-completion/completions/proxmox-backup-client
- /usr/share/bash-completion/completions/pxar