various bug fixes

• support qrexec protocol version 3 (@reynir @palainp in mirage-qubes 0.9.3)
• remove special DNS rule (which used to be required for Qubes 3, issue #63, fix #142, @hannesm)
• use DNS servers from QubesDB instead of hardcoded ones for evaluation of the DNS rule (#142 @hannesm)
• remove the GUI code (not needed in Qubes 4.1 anymore, issue #62, fix #144, @palainp)
• trigger GC slightly earlier (at < 50% free space, issue #143, fix #147, @palainp)