re-implement daemonize (#8011)

This has long been something I've wanted to do. Basically the `Daemonize` code
is both too flexible and not flexible enough, in that it offers a bunch of
features that we don't use (changing UID, closing FDs in the child, logging to
syslog) and doesn't offer a bunch that we could do with (redirecting stdout/err
to a file instead of /dev/null; having the parent not exit until the child is
running).

As a first step, I've lifted the Daemonize code and removed the bits we don't
use. This should be a non-functional change. Fixing everything else will come
later.