if: Add compat mode for ifnet stats so that 3rd party apps can work

If ifnet stats compat mode is enabled, the per-CPU ifnet stats will
be collected into ifnet old stats location in the if slow timer.

Sysctl node net.link.stats_compat is used to control whether the
ifnet stats compat mode should be enabled or not; it is _not_ enabled
by default.
icmp: Disable icmplim_output by default

It spams the console a pretty lot.
bce: Factor out bce_xmit()
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.
vkernel - Allow setting MAC addresses from within the command line.

  In order to be able to specify the MAC address we want to
  use for every interface within the vkernel, an extra argument
  has been added to the -I option.

  Example:
  	./kernel -r root.img -m 256m -I auto=aa:bb:cc:dd:ee:ff
bce: Put TX related fields into bce_tx_ring