Handle requests with no attributes correctly in the test server.
Add basic RFC 3576 support based on the work of Gabriel Blanchard.

This adds basic support for CoA and disconnect messages, along with the
required constants for the new message and attribute types.
Add support for sending tags in non-vendor attributes.
Add untested support for vendor attribute tags.
Password attributes don't support tags, obviously.
Hexdump the error output when an attribute match fails for easier reading.
Add test server support for vendor specific attributes.
Fix VS attribute size handling.
Add vendor specific attribute setter tests.