Cleanup systemd files

- Added documentation lines for each service
- Added Restart on-failure for each service
- Remove Alias from -dir (only for .target unit)
- Added soft dependency for postgresql & mysql for bareos-dir in
  After= line so systemd know we prefer to start once the db is up

Fixes #240: Improve systemd service files
Add comment for encryption support.

Some libraries may be setup for Library based encryption or for usage of
a Key Management Appliance and that may prohibit the use of Application
Managed Encryption that our scsi-crypto plugin implements. So in those
situations you need to disable the LME or KMA settings of the library.