Open files after daemonizing

When daemonzing, all open files are closed. It is possible to whitelist
files to be kept open, but I couldn't find a way to get the file
descriptor. Therefore, the following are now done after daemonization:

 - Import (and therefore instantiate) the database
 - Start logging
Log to stderr when run in foreground
Don't put the OAuth client in the app context

Since that apparently doesn't work, and the performance impact of
doing it on every request is negligible.
Add support for running server in foreground to the usage message