cellconfig: check for invalid dotted quads

cellconfig: check for invalid dotted quads

IP addresses entered into the CellServDB with components larger
than 255 would silently be trucated down to 8-bit unsigned integer
representations.  This could cause confusing behavior with
occasional hangs.

FIXES 131794

Reviewed-on: http://gerrit.openafs.org/12109
Reviewed-by: Chas Williams <3chas3@gmail.com>
Reviewed-by: Michael Meffie <mmeffie@sinenomine.net>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
(cherry picked from commit 97150150e6d12cbbc0c4a5af3424c9bf1e56918c)

Change-Id: I4e628ab7e12e33b23cc513a268879de115ddec2e
Reviewed-on: https://gerrit.openafs.org/12210
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Mark Vitale <mvitale@sinenomine.net>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>