scanner: enforce correct argument type for enums

The scanner now checks whether arguments that have an associated
<enum> have the right type.
An argument with an enum attribute must be of type int or uint,
and if the <enum> with that name has the bitfield attribute
set to true, then the argument must be of type uint.

Changes since v3:
 - Remove useless allow_null check
 - Switch to using bool
 - Clearer message on errorous input
 - Minor formatting fix

Signed-off-by: Auke Booij <auke@tulcod.com>
Reviewed-by: Bryce Harrington <bryce@osg.samsung.com>
Reviewed-by: Nils Chr. Brause <nilschrbrause@googlemail.com>