libsmack: add a common function for opening long and short label file

Added internal function 'open_smackfs_file()' to open long label
file and as a fallback short label file. This can be used for
load, access and cipso files.

Signed-off-by: Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>
libsmack: fallback to 'cipso' when 'cipso2' is not available

This patch implements fallback to 'cipso' when 'cipso2' is not
available. This is a regression from 79f8e26.

Signed-off-by: Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>