* Replace {Set|Remove}MasterKey() by generic {Set|Remove}UnlockKey()
that works on a keyring.
* Implement {Set|Remove}MasterUnlockKey() on top of that.
* Rename the commands and constants accrodingly.
* Implement setting and removing keyring unlock keys.
* The keystore backend will (at least for the time being) reside in a
separate server. This one can be reached via normal messaging, so use
a BMessenger for sending key messages.
* Move the message constants from RegistrarDefs.h into a new
KeyStoreDefs.h that also contains the server signature.
* Update the message constants to reflect the new situation.
