KEYS: DigitalSignature link restriction

Add a new link restriction.  Restrict the addition of keys in a keyring
based on the key having digitalSignature usage set. Additionally, verify
the new certificate against the ones in the system keyrings.  Add two
additional functions to use the new restriction within either the builtin
or secondary keyrings.

[jarkko@kernel.org: Fix checkpatch.pl --strict issues]
Signed-off-by: Eric Snowberg <eric.snowberg@oracle.com>
Reviewed-and-tested-by: Mimi Zohar <zohar@linux.ibm.com>
Reviewed-by: Jarkko Sakkinen <jarkko@kernel.org>
Signed-off-by: Jarkko Sakkinen <jarkko@kernel.org>