Need better way of listing Serial ports on Linux.

[wb8tyw]

The existing way of identifying Linux serial device is not finding all serial devices as it is relying on a hard coded list of device prefixes.

On newer Linux systems we should read the symlinks in /dev/serial/by-path if that directory exists.

Need to see if the /dev/serial exist in MAC OS or in linux emulation on Microsoft Windows.

[wb8tyw]

This is needed as some radios may now be using bluetooth serial ports.

https://gist.github.com/0/c73e2557d875446b9603