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.
build - Do not use cp -a

* The -a option to cp was added in November, unbreak installworld for those
  running a world built prior to November 2012.
csu: Fix .eh_frame_hdr errors seen on i386

The libcsu object files should have been generated with
-fno-asynchronous-unwind-tables.  The crtbegin*, crtend* objects,
specific to a compiler, were generated with this flag on both platforms
as seen in the vendor build.  This commit builds libcsu with the same
cflags on both platforms, and it allows the error frame header to get
built successfully.

This has been seen on i386 for a while, and later bug #2511 hit upon
it outside of the world build.