The signal return code was trying to access user mode addresses

The signal return code was trying to access user mode addresses
directly, which doesn't work when the user process is in an entirely
different vmspace.  Use copyin() instead.

Assign SIGQUIT (^\) on the console to cause a virtual kernel to enter
its internal db> debugger.

Quickly hack a callout() based poller for console input so the console
works (sorta).

Allow a kernel environment to specified as an option in the form
-e name=value:name=value:...

NULL-out the pmap self-mapping cache when a pmap is destroyed.