Add journaling restart support, required to produce a robust journaling

Add journaling restart support, required to produce a robust journaling
environment.  If a journal is writing to one stream and the stream breaks
or dies or otherwise fails, this feature gives us the ability to restart the
journaling stream on a new descriptor without losing any data.  The
journaling restart code does a shutdown() of the old descriptor, waits for
both directions to cease operation, installs a new stream descriptor,
and resets the FIFO index to the last acknowledged offset.

This can be demonstrated by opening two windows.  In the first window do:

    mountctl -a2 /usr:test | jscan -d2 stdin

Mess around a bit on /usr.  Then in the second window do:

    mountctl -r2 /usr:test | jscan -d2 stdin

The first jscan will terminate and the new jscan will pick up the stream.