veyron2/vlog: add vlog.Panic{,f}
veyron2/vlog: add vlog.Panic{,f}

This is useful as shorthand for vlog.Error() followed by log.Panic(),
and makes it so developers don't have to import "log" for this
usecase.

Furthermore, currently we have various main()'s that call log.Fatal
after one or more defer's, which is dangerous in that it may leave
garbage behind (since log.Fatal calls os.Exit, which does not run
deferred functions before exiting). In many such cases vlog.Panic may
be more appropriate.

Change-Id: Ibfd2188fd694d9c75484e1b022acec2c1bbd8527

vanadium/go.v23,vanadium/go.v23,vanadium/go.v23