vlog: Add VLOG_ABORT() to log and call abort().

vlog: Add VLOG_ABORT() to log and call abort().

Whereas VLOG_FATAL() eventually calls exit(1), VLOG_ABORT()
eventually calls abort().  The key difference is that abort()
will cause a "monitor" process to restart, where exit(1) will
cause it to exit along with the monitored process.

Signed-off-by: Ben Pfaff <blp@nicira.com>