USB tethering between FreeBSD and mobile phone (Debian OS)

Hi Mathias,

I added a single line in `/etc/rc.conf` to take care of this 
automagically when I need to tether via USB to my Pixel running 
GrapheneOS (which also produces a `ue0` interface on the FreeBSD side):

ifconfig_ue0="up SYNCDHCP"

IIRC, I found this solution here:
https://forums.freebsd.org/threads/wireless-setup-and-syncdhcp.41488/

I'm usually not tethering so it thus causes a slight delay at boot 
because the interface isn't up at that point and boot is paused until 
this times out, but it causes `dhclient` to remain running and fetch an 
address whenever it does come up.

It's possible this isn't the best way to do it; perhaps others can chime 
in to improve further on this implementation.

Cheers,
Alex