[NNTP] Command names

[maurizio codogno]

>>>I wonder if it would be a good idea to limit both extension labels
>>> and command names to some smaller character set (say alphanumeric 
>>>plus dot and dash)?

>Okay, here's the proposal. Any comments or objections?

fine for me. 
Underscore could have been fine, but I see no mandatory use for it,
so it's better to stick to alphanum plus dot and dash.

ciao, .mau.