make owner of the backup device configurable

on OpenBSD it should be '_bacula-sd' though elsewhere 'bacula' suffices
Add support for OpenBSD (dir, sd and fd)
rework how jobs use a fileset, allowing to use a fileset defined elsewhere
Merge pull request #16 from jasperla/device_owner

make owner of the backup device configurable
Merge pull request #18 from jasperla/jobs

adjust how jobs define filesets
Merge pull request #17 from jasperla/obsd

Add support for OpenBSD (dir, sd and fd)