hdt: Execute hdt default callbacks in each mode if needed

hdt: Execute hdt default callbacks in each mode if needed

Impact: One can switch modes without going back through hdt

Given a command `dmi> show foo', the ordering of callbacks is now
the following:
	1/ execute module 'foo' of commands set 'show' of mode dmi
	2/ execute default callback of commands set 'show' of mode dmi
	3/ [NEW] execute module 'foo' of commands set 'show' of mode hdt

(2/ is executed if 1/ doesn't exist, 3/ is executed if 2/ doesn't exist
either, if 3/ doesn't exist an error is displayed).

This allows for instance to switch between modes without exiting first
to hdt.

Signed-off-by: Pierre-Alexandre Meyer <pierre@mouraf.org>