daemon: Integrate checking for an existing pidfile into daemonize_start().

Until now, it has been the responsibility of an individual daemon to call
die_if_already_running() at an appropriate time.  A long time ago, this
had to happen *before* daemonizing, because once the process daemonized
itself there was no way to report failure to the process that originally
started the daemon.  With the introduction of daemonize_start(), this is
now possible, but we haven't been taking advantage of it.

Therefore, this commit integrates the die_if_already_running() call into
daemonize_start() and deletes the calls to it from individual daemons.