RPM spec updates

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The RPM spec doesn't seem to be up to date and it fails to build packages at the moment.

The updated spec file:

• Uses the install Makefile rule instead of install-strip, which isn't working properly
• Adds nullmailer-dsn to the package
• Replaces supervise-scripts with systemd
• No longer removes the nullmail user and group on uninstall, so that leftovers can't be accessed by random stuff that gets installed later