Use the lckpwdf() again if prefix is not set

Use the lckpwdf() again if prefix is not set

The implementation of prefix option dropped the use of lckpwdf().
However that is incorrect as other tools manipulating the shadow passwords
such as PAM use lckpwdf() and do not know anything about the
shadow's own locking mechanism.

This reverts the implementation to use lckpwdf() if prefix option
is not used.