Implement SIGTERM handling. When a SIGTERM is received by a VKERNEL, it

Implement SIGTERM handling.  When a SIGTERM is received by a VKERNEL, it
will automatically shutdown.

A little shutdown kernel thread is created to handle the shutdown, since
an operation at that level of significance requires a real thread context
as a base of operations.

Submitted-by: Chris Turner <c.turner@199technologies.org>