gnome-keyring Review: Give daemon access to collection credentials

[Stef Walter <stefw collabora co uk>]

Till now the master secrets for the various keyrings ('collections' in 
Secret Service API parlance) have been hidden away in the secret-store 
module. These are held in CKA_VALUE PKCS#11 attiribute on objects of 
type CKO_G_CREDENTIAL, and are used to unlock the keyring objects.

Until now any attempt to read out these master secrets has resulted in 
the error code CKR_ATTRIBUTE_SENSITIVE.

However, since we'd like to be able to use these master secrets to do 
things like NTLM. The daemon needs access to read them.

This patch adds access for the daemon to read them via PKCS#11. If an 
application tries to access them, then the app will still get 
CKR_ATTRIBUTE_SENSITIVE.

I've added tests to verify this.

Patch (and discussion) here: 
https://bugzilla.gnome.org/show_bug.cgi?id=652070

Any review of this code possible is appreciated. I'm available to help 
fill in any gaps if something's not clear.

Cheers,

Stef