Get signature version from the header

Get signature version from the header

Currently we assume signature version is v1 until and unless -x is
specified on kernel command line. Given the fact that signature version
information is available in signature itself, it is much better to get
it from there and not require user to pass -x during verification phase.

If user passed -x on command line, then honor it.

Now one can do following.

	evmctl ima_sign -x /tmp/data.txt
	evmctl ima_verify /tmp/data.txt

Signed-off-by: Vivek Goyal <vgoyal@redhat.com>