Avoid gcc warning

Avoid gcc warning

When using the configure option --enable-checking with gcc 7.2.1,
the compilation fails with

vutil.c:860:20: error: ‘%s’ directive writing up to 255 bytes into \
a region of size 63 [-Werror=format-overflow=]

This can be seen in the logs of the openSUSE Tumbleweed builder
for e.g. build 2368.
Avoid this warning by using snprintf which is provided by libroken
for all platforms.

Reviewed-on: https://gerrit.openafs.org/12813
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
(cherry picked from commit fd4eaebb60dbefc27be98015fee23a3cf5d9752d)

Change-Id: I3be14f6f1228fd09f036da7ff4f1505c65e49406
Reviewed-on: https://gerrit.openafs.org/12897
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>