passwd: Unify (un)locking routines

passwd: Unify (un)locking routines

Make sure that passwd and shadow are always opened in the correct
order to avoid possible dead locks with other tools:

- Lock passwd first, then shadow
- Unlock shadow first, then passwd

The passwd utility may work without a shadow entry. In that case, it
operates on the passwd file. But to figure this out, the shadow file
must have been opened and thus locked already. Unconditionally open the
passwd file first, even though it's not needed most of the time.

Reviewed-by: Alejandro Colomar <alx@kernel.org>
Signed-off-by: Tobias Stoeckmann <tobias@stoeckmann.org>