Fix showing build steps in the machine status page

This was broken because of the comparison

  step.machine.match('@(.*)').0 == m.key

where m.key contains the SSH user.

Also some style tweaks.
Machine status: Don't show removed machines anymore

This requires a sequential scan on the BuildSteps table, which by now
takes > 8s.
Remove some redundant SQL queries
Make negative narinfo responses much smaller
Minor tweak
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.