Make clear in the ipheth(4) hardware notes that this driver is for the
tethering functionality only.  Add a "bugs" section to give a pointer
to usbconfig set_config if the device isn't automatically detected.

MFC after:	3 days
Add a missing file from the last commit.

Noticed by: jhibbits
When plen != ulen, it should only be checked when this is UDP.

Spotted by:	bryanv