util: add afs_exec_alt

util: add afs_exec_alt

Add the function afs_exec_alt to help programs easily execute an
"alternate" version of themselves. For example, for programs that are
built with/without DAFS support, or are built for 32-bit/64-bit
structures, etc.

Change-Id: Ibb2b7105d58476f84bd9f15987f8b7df37314b6b
Reviewed-on: http://gerrit.openafs.org/2483
Tested-by: Andrew Deason <adeason@sinenomine.net>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>