build: add "make backupworld" and "make restoreworld" functionality

build: add "make backupworld" and "make restoreworld" functionality

Three new make.conf parameters have been defined:
  * WORLD_BACKUP - location to store backed up world binaries
                   default = /var/backups/world_binaries
  * DAYS_BACKUP  - The number of days since the last backup that must pass
                   before "make installworld" fails with an error
                   default = 28
  * NO_BACKUP    - defining this will prevent backup checks.

The build functionality has been changed.  Prior to "make installworld",
the makefile will check to see if system binaries have been previously
backed up.  If they haven't, "make installworld" will fail to execute
explaining that the system should be backed up.  If a previous backup
does exist, but it's older than the specified number of days, "make
installworld" will fail explaining the backup needs to be refreshed.

Passing NO_BACKUP through the command line or make.conf will inhibit
these checks.

While here, define WORLD_CCVER, LDVER, WORLD_LDVER, WORLD_BINUTILSVER
in make.conf man page too.