Debian fixes

Dear Developers,

These are fixes for bugs found in Debian packages that can be applied to the buildbot repository:

 * e79649a adds possibility to restart a sigle master/slave from init script
 * daa4841 switches on discarding output using /dev/null redirection since `--quiet` option makes startup always exit with zero exit code
 * 1cba0fd will directly point user to fix `buildbot.tac` file and retry upgrade (old message was just wrong — although non-zero exit code indicates a problem, output shows only warning despite database not being updated)