ACPICA: Debugger: add "background" command for method execution

ACPICA commit d7b44738a48caa9f669b8dbf0024d456711aec31

Allows a single task to execute in the background, while control
returns to the debugger prompt.

Also, cleanup the debugger help screen.

Link: https://github.com/acpica/acpica/commit/d7b44738


Signed-off-by: Bob Moore <robert.moore@intel.com>
Signed-off-by: Erik Schmauss <erik.schmauss@intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>