evm: Enforce signatures version 3 with new EVM policy 'bit 3'

Enable the configuration of EVM so that it requires that asymmetric
signatures it accepts are of version 3 (sigv3). To enable this, introduce
bit 3 (value 0x0008) that the user may write to EVM's securityfs policy
configuration file 'evm' for sigv3 enforcement.

Mention bit 3 in the documentation.

Signed-off-by: Stefan Berger <stefanb@linux.ibm.com>
Signed-off-by: Mimi Zohar <zohar@linux.ibm.com>