dictionary: Make dict_create() take the new dictionary's encoding.

dictionary: Make dict_create() take the new dictionary's encoding.

There are several places in the PSPP tree that create dictionaries,
but few of them actually set an encoding.  This causes most
dictionaries to be in the default encoding, which is often not
correct.

By making dict_create() take the encoding as a parameter we force
the caller to think about the encoding issue up-front.