Make sure reboots work correctly

We should at least have an optional mode that ensures that the current build finishes before the reboot proceeds - and no further builds are started. Right now pybuildd will DTRT when it receives SIGUSR1: it will eventually exit, i.e. after finishing the current build in progress. We could hook this up in systemd to actually wait for the current build to finish. This would help on slow architectures like riscv64.

One complication is that the system systemd needs to wait for the user session to finish up.