Decode encrypted VSAs in requests

Decode encrypted VSAs in requests

Incoming CoA requests can contain encrypted VSAs.  At least one
vendor is known to use this. These VSAs must be decrypted before
being proxied to enable the server to re-encrypt them using
the correct home server secret.

Fix by attempting to decode any encrypted request attribute using
a static vector of \0 bytes.

This also fixes debug logging of encrypted request attributes.

Signed-off-by: Bjørn Mork <bjorn@mork.no>