por-file-reader: Remove dependency on VAR_NAME_LEN.

por-file-reader: Remove dependency on VAR_NAME_LEN.

VAR_NAME_LEN wasn't really needed here because we knew that the name we
started from was no more than 8 bytes long.  Also, we know that we can
come up with a unique name within ULONG_MAX tries since we'd run out of
memory before running out of values to try.

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.