redhat: Use the right path to depmod

As of Fedora 17 and RHEL 7, depmod has moved from /sbin to /usr/sbin.
The full path to depmod is used in package scripts and as a dependency.
This hasn't caused problems in most cases because on an installed
system a link /sbin -> /usr/sbin is present and during ordinary package
installations yum/rpm correctly then figure out that /sbin/depmod is
actually provided. But in other situations, the dependency check is not
that clever and (incorrectly) fails.

Add a macro to the spec defining the full path to depmod, use the macro
rather than plain /sbin/depmod throughout the spec, and also pass it to
kmodtool when required to generate the kmod package scripts and
requirements.

FIXES 131860

Reviewed-on: http://gerrit.openafs.org/11128
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Reviewed-by: Perry Ruiter <pruiter@sinenomine.net>
Reviewed-by: D Brashear <shadow@your-file-system.com>
(cherry picked from commit c20c01185ed748b2bc823369a8f28cf004b7d1c9)

Change-Id: Ib9e0bfe586de668ddba6fb82ff1ea2a081277150
Reviewed-on: http://gerrit.openafs.org/11171
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Perry Ruiter <pruiter@sinenomine.net>
Reviewed-by: Ken Dreyer <ktdreyer@ktdreyer.com>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>