logfile: Do not log differently with --nodetach

This is actually quite anoying.  There is no good reason for leaving
off the timestamps in tests, or manual debugging, etc.

If at some point we want to support running under some kind of
log-capturing daemon supervisor which timestamps everything, we can
easily make this conditional again.

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>