upserver: Don't overflow file and hostname buffers

upserver: Don't overflow file and hostname buffers

If the user specifies a ridiculously long command line, don't
overflow the filename or hostname buffers with what they supply.

Caught by coverity (#985911)

Change-Id: Ia73f9fb94491f5691358eec1d13dbdd2651a604c
Reviewed-on: http://gerrit.openafs.org/9546
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>