cli: allow specifying sub commands through $cmddef

allow to use sub commands alá
 # pveum user add

The new resolve_cmd traverses $cmddef, resolves one level of aliases
and returns the respective sub command, its cmddef, arguments and if
it was expanded (e.g., pveum u d ... => pveum user delete ...) which
allows quite easy integration in the usage/synopsis generator, bash
completion helper and command handler.

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>