modify log_file option

Gunicorn should generally only bother writing its own log and let
the application handle the way it want to log its own errors.

Now the log_file option will be overriden by the gunicorn options
`--error-logfile` and `--access-logfile` if they are given.
don't propagate logs

fix #574
don't try to redirect stdout/stderr to the logfile.

Logging to a logfile should be explicit and gunicorn should not try to
be smarter here. Partially fix #591
make it explicit that we only send gunicorn logs to syslog.

follows suggestion in #548
allows possibility to set the driver used in syslog for unix sockets

This changes allows a use to set the driver used to send the data over a
unix socket. 'dgram' for a dgram driver or 'stream' for a stream driver.
'stream' is the default. The change is documented in the settings
documentation.

fix #671