Improve systemd service file

[ppacher]

This PR updates the systemd service to include more protective stanzas.

Related Issues

• https://github.com/safing/portmaster/pull/96 is required for this
• https://github.com/safing/portmaster/issues/99 is affected as we have a Conflicts= stanza now.
• https://github.com/safing/portmaster-packaging/issues/2 is fixed