implements set_interface

[philrhc]

if selected interface is a single digit, then it is used as an array index in the network interface list
otherwise the selected interface is attempted
otherwise the default behaviour of looping through the network interface list, selecting the first eligible.

this continues the closed pull request here: #164

[sphaero]

thought of anyway to test this?

[philrhc]

@sphaero

thought of anyway to test this?

i've only been testing this locally using the chat example app

[philrhc]

i will do some further testing in our application
for automated testing, i am not sure, i think your suggestion is best but it is hard for me to prioritise it:
#164 (comment)