vswitchd: Improve log options passed down to secchan.

vswitchd: Improve log options passed down to secchan.

Previously, secchan as started by vswitchd would always log to the console.
Now, it logs to the console only if any logging to the console is enabled
for vswitchd.

Previously, secchan as started by vswitchd would never log to a file.  Now,
it logs to a file if vswitchd is configured to do so.

Also, use the bridge name in the vlog log pattern, instead of the datapath
number, so that users can more easily relate log messages to bridges.

Requested by Keith.