util: Fix exec_alt

util: Fix exec_alt

exec_alt was failing its tests on Mac OS X (but passing them on Linux).

It turns out that this is because it was failing to NULL terminate the
string that it creates in construct_alt(), which copies in the
characters from argv0, prefix and suffix, but never copies in a trailing
NULL.

Amend the code so that the trailing NULL from suffix is used to
terminate the string.

Change-Id: I286604fb09193367c4e7b2d80051832080362f79
Reviewed-on: http://gerrit.openafs.org/4267
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Derrick Brashear <shadow@dementia.org>