init: add 'enable <service>' to negate "service <service>... disabled"

enable <servicename>
   Turns a disabled service into an enabled one as if the service did not
   specify disabled in the rc file.
   It will also start the service if needed.

Bug: 14472973
Change-Id: Id0b49cc687a2bc74f6f92e066c617724cc94908d
Signed-off-by: JP Abgrall <jpa@google.com>