Add generic unlock key setting and removal.

* 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.

Rename keyring "access/revoke" to "unlock/lock".

The unlock/lock concept just seems easier to grasp and is used in
various similar tools as well.

Remove unneeded master access revoke command.

Revoking master access currently simply means to revoke access
to the default keyring.

Move keystore message constants and use a messenger.

* 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.

Add generic unlock key setting and removal.

* 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.

Rename keyring "access/revoke" to "unlock/lock".

The unlock/lock concept just seems easier to grasp and is used in
various similar tools as well.

Remove unneeded master access revoke command.

Revoking master access currently simply means to revoke access
to the default keyring.

Move keystore message constants and use a messenger.

* 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.