KEYS: Add a 'trusted' flag and a 'trusted only' flag

Add KEY_FLAG_TRUSTED to indicate that a key either comes from a trusted source
or had a cryptographic signature chain that led back to a trusted key the
kernel already possessed.

Add KEY_FLAGS_TRUSTED_ONLY to indicate that a keyring will only accept links to
keys marked with KEY_FLAGS_TRUSTED.

Signed-off-by: David Howells <dhowells@redhat.com>
Reviewed-by: Kees Cook <keescook@chromium.org>