hydra-eval-guile-jobs: Use `--fresh-auto-compile'.

* src/script/hydra-eval-guile-jobs.in (main): Run `guile' with
  `--fresh-auto-compile'.
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.