Make "internal variables" part of a dictionary.

Make "internal variables" part of a dictionary.

The "internal variables" created by var_create_internal() are somewhat of
a nasty corner case in the PSPP dictionary and variable implementation,
because they are the only variables that have a case index and
dictionary index but do not belong to a dictionary.  This commit fixes
that wart, by putting internal variables in a dictionary that is used just
for them.  It adds an assertion to var_set_vardict() to ensure that this
is now an invariant property.

As part of the change, var_create_internal() is renamed
dict_Create_internal_var() and moved to dictionary.c, since it fits better
there.  Also, a new function dict_destroy_internal_var() must now be used
to destroy internal variables.

The dictionary indexes of internal variables are no longer unique
process-wide.  A previous commit eliminated the dependencies on this
property.