uri: Make vendor query attribute handling reliable

uri: Make vendor query attribute handling reliable

Previously we used p11_dict to keep track of vendor query attributes.
This had a couple of limitations: duplicate attributes are not allowed
while they are actually allowed in RFC 7512, and the order of
attributes is unpredictable.

This patch switches to using an array instead of p11_dict and ensures
that the attributes are sorted in alphabetical order.

Fixes #88.