salvageserver: Add -logfile option

Allow an alternate location to be specified for the logfile.

Change-Id: I55a05576746e115478a8e48df22f4fdb26634f05
Reviewed-on: http://gerrit.openafs.org/9831
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>