LINUX: ifconfig is deprecated

LINUX: ifconfig is deprecated

ifconfig is deprecated and is no longer installed by default on RHEL 7 and
Centos 7. Use the replacement ip command in the init script for linux.
Fallback to ifconfig in the event the ip command is not available.

Thanks to Ben Kaduk for pointing out the hash built-in command.

Reviewed-on: http://gerrit.openafs.org/12192
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Chas Williams <3chas3@gmail.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
(cherry picked from commit b702ab5da216976ed01ad3b1c474ecd4cc522ff2)

Change-Id: I9ffdfee233555f1e06bc4f980e2905851224ecc9
Reviewed-on: https://gerrit.openafs.org/12193
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Mark Vitale <mvitale@sinenomine.net>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>