Makefile.inc: Rework automatic backup (bug 2527)

There were three main problems with the recently added backup feature:
1. It didn't work over NFS
2. It didn't work when a non-root built world the second time
3. It didn't use DESTDIR which had potential to conflict backups

It also previously featured "NO_BACKUP" which turned off the automatic
backup feature.  To fix the #2 problem, the install timestamp has been
removed, so to avoid unwanted backups for multiple installworlds after
one buildworld, the user needs to pass NO_BACKUP in the command line.

This patch was contributed by Thomas Nikolajsen with minor modifications
added by John Marino.

http://bugs.dragonflybsd.org/issues/2527