Add argument checks to policy <allow>/<deny> rules.

[Pekka Pessi]

Hi all,

The patches add send_signature*, send_arg*, receive_signature, and
receive_arg* attributes to the <allow> and <deny> policy rules. With the
additions it is possible to restrict access to certain methods and signals
based on the argument values.

For example, oFono uses extensively SetProperty methods with signature sv (s
for property name, v for value), pretty much like o.f.D.Properties.Set. The
argument-based policy can control write access to individual properties.

The first two patches just simplify append_rule_from_element() and
locate_attributes () functions in bus/config-parser.c.

Have a peek in the patches and please push if they seem to be sane enough.

--Pekka Pessi