salvager: Do not abort on large volume IDs

salvager: Do not abort on large volume IDs

The salvager was parsing volume IDs just using atoi() and checking if
the result was negative. Since the result is a signed int, this fails
on any volume ID larger than 2^31-1. Change the parser to use strtoul
instead of atoi, and change the check.

While we're here, make a similar change to the DAFS salvageserver,
too.

Change-Id: Icc3377ee507150ff0c53b5bbff6172cb72bca703
Reviewed-on: http://gerrit.openafs.org/4196
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementia.org>