Revision 277297

MFC r276901:
  Move the recursion detection code into separate function
  gif_check_nesting(). Also make MTAG_GIF definition private to if_gif.c.

MFC r276907:
  Restore Ethernet-within-IP Encapsulation support that was broken after
  r273087. Move all checks from gif_output() into gif_transmit(). Previously
  they were checked always, because if_start always called gif_output.
  Now gif_transmit() can be called directly from if_bridge() code and we need
  do checks here.

  PR:		196646