Create Unix installer

[zombiezen]

The build server requires some setup on Unix systems:

• Create build user and groups
• Arrange for a suitable /bin/sh to be present in the Linux sandbox
• Install a unit in systemd

There should be:

• An automatic script that installs zb for the common case.
• An automatic script that uninstalls zb after the first script has been run.
• Documentation for system administrators that clearly describes what the installer does, and what zb needs for a custom installation.