build: implement automatic world backups

build: implement automatic world backups

The directives DAYS_BACKUP and NO_BACKUP have been removed.
The "backupworld" target will save important directories to the WORLD_BACKUP
directory just as before, and it is restored with the "restoreworld" target.

Additionally, every time the "installworld" target is executed, the same
directories will be automatically backed up at the location of
${MAKEOBJDIRPREFIX}/world_backup .  These directories could be restored
with the new make target "restoreworld-auto".

The WORLD_BACKUP location default is now /var/backups/world_backup .
The directory /usr/lib has been added to the backup list.
The more useless errors seen with a broken world have been removed, these
came in with bmake.