bos: Improve string safety

bos: Improve string safety

To avoid potential string overflows and allow for larger strings in the
future, convert to safe string functions and remove fixed length stack
allocated strings in the bos client.

Add string truncation checks to the date format helper function.

Change-Id: Icf73b970d2c5d22fc6d3c8201498eff74e104812
Reviewed-on: https://gerrit.openafs.org/14765
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>