Merge r204173 from head to stable/8:

Merge r204173 from head to stable/8:

  ifconfig(8) expects interface fooX to be supported by the module if_foo,
  and will try to load it if it's not present.  To better meet these
  expectations, change the module name for the loopback interface from
  'loop' to 'if_lo'.  The loopback interface is always compiled into the
  base kernel, so there are no resulting changes in kld files, etc.

  Discussed with:       brooks (ages ago)

Approved by:    re (bz, kib)

git-svn-id: svn://svn.freebsd.org/base/stable/8@208679 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f