viced/callback.c: Don't ignore dump read errors

viced/callback.c: Don't ignore dump read errors

When reading a callback state dump, check the return values from
read(2) instead of ignoring them.  This adds a new static function,
ReadBytes(), which handles reading a requested number of bytes from a
file and bailing if there is an error.

Reviewed-on: http://gerrit.openafs.org/9989
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
(cherry picked from commit c854cb31ff2b1ab79de18b5ab926bf2ea2b05663)

Change-Id: Ieffa11c6049eceb23bc6bf9169454529df3ef766
Reviewed-on: https://gerrit.openafs.org/13505
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Cheyenne Wills <cwills@sinenomine.net>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>