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

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:		[[ https://bugs.freebsd.org/196646 | 196646 ]]