Merge pull request #84 from peti/update-hydra-check-space-for-systemd

hydra-module.nix: update hydra-check-space for systemd
Fix the test
hydra-module.nix: find sendmail in /var/setuid-wrappers
hydra-module.nix: strip trailing whitspace
hydra-queue-runner: don't clutter the system log with debug message

Avoid the frequently printed

  hydra-queue-runner[10293]: system type `x86_64-linux': 2 active, 2 allowed, starting 0 builds

message. That information is only interesting when some build are
actually started.