Install with new tar.xz extension

[Robert Nestor]

Tried downloading and installing 8.99.25 this morning and noticed that it fails 
because the installation utility (sysinst) doesn’t support the new tar.xz 
extension.  PR pkg/53697 seems to address this in the sysutil/sysupgrade 
utility.  Is another PR needed to get this addressed in the sysinst utility?