vswitchd: Integrate secchan into vswitchd.

vswitchd: Integrate secchan into vswitchd.

This change means that vswitchd is now a single process that manages all
of the datapaths, instead of a parent process and any number of secchan
subprocesses.

It may be better to fork off processes that each manage a few bridges, to
take advantage of multiple cores.  Perhaps this is upcoming.

There are numerous new configuration file directives that are, sadly, not
yet documented.