venus: convert fs.c to safer string functions

venus: convert fs.c to safer string functions

Convert string handling to safer functions to avoid buffer overflows.

Reviewed-on: https://gerrit.openafs.org/12923
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
(cherry picked from commit 5a8b68153124c3a9224f0b6993df9de9c6c54541)

Change-Id: Ic7fd4c3c0ffd4b0b38b9735002e9bf8eba8fd0d6
Reviewed-on: https://gerrit.openafs.org/13100
Reviewed-by: Michael Meffie <mmeffie@sinenomine.net>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Tested-by: BuildBot <buildbot@rampaginggeek.com>