KEYS: Make the system 'trusted' keyring viewable by userspace

Give the root user the ability to read the system keyring and put read
permission on the trusted keys added during boot.  The latter is actually more
theoretical than real for the moment as asymmetric keys do not currently
provide a read operation.

Signed-off-by: Mimi Zohar <zohar@us.ibm.com>
Signed-off-by: David Howells <dhowells@redhat.com>