REGRESSION: Eliminate restriction to VAR_NAME_LEN in reg_get_name().

There's still an obvious problem here that the prefix isn't being trimmed
down so that the suffix will fit.  Since an upcoming series of changes
would have to completely redo how this would be done, I'm not fixing that
now, only marking it with XXX.

Most uses of VAR_NAME_LEN within PSPP are wrong due to encoding issues:
the limit applies to variable names in the encoding used by the data
set, but most uses of VAR_NAME_LEN actually limit the length of a name
in UTF-8.  The UTF-8 representation of a name can be longer or shorter
than its representation in the data set encoding, so it seems best to
eliminate references to VAR_NAME_LEN entirely.