Add an example devpubd hook that makes stable path names, using symlinks,
for USB serial interfaces, regardless of where the interface is connected
or the order of enumeration. This requires the USB device to have a
"serialnumber" to function. Examples:

- uftdi serial adapter with two ports:

/dev/tty-uftdi-FT64S4YP-1 -> /dev/ttyU0
/dev/tty-uftdi-FT64S4YP-2 -> /dev/ttyU1

- uslsa serial adapter:

/dev/tty-uslsa-01E7ABCC -> /dev/ttyU4

This allows something like the following in /etc/remote:

sun3:dv=/dev/tty-uftdi-FT64S4YP-1:br#9600:pa=none:dc:

That path will always be stable regardless of which /dev/ttyU* node is
actually assigned when the serial adapter is plugged in.