Don't pad variable labels written to system files.

Don't pad variable labels written to system files.

Previously, the code to write variable labels to system files accidentally
padded out the variable labels on the right with spaces to a multiple of 4
bytes in length.  With this commit, variable labels will no longer be
padded.

This fixes a genuine bug for variable labels longer than 252 bytes, which
previously were padded out to 256 bytes.  Variable labels are limited to
a maximum of 255 bytes, so PSPP refused to read such files.

Reported-by: Robert Westlund <rob@kinesissurvey.com>