libsmack: add support for new access mode for setting locks ("l")

This change should be backward compatible for kernels without l-mode support
as long as requested permissions don't contain this mode.
Note: ACC_CLEAR is intentionally not changed. It is assumed that kernels
without load2 smackfs interface won't support this access mode either.

Signed-off-by: Rafal Krypa <r.krypa@samsung.com>