var: fix segv in deprecated file var show_help()

Ensure to include the new variable filename in the show_help() output
when we load a deprecated MCA param from a file.

Fixes #236

(cherry picked from commit dc66e197cc48396002d0dcf6f5f04dc6b581d7fd)
Merge pull request #33 from jsquyres/topic/var-show-help-segv-fix

var: fix segv in deprecated file var show_help()

RM-approved
Correctly move the pointer back by the true_lb.

(cherry picked from commit open-mpi/ompi@e3f74aca1c0482816e85da9f018872b7fe98e227)