Add basic encryption detection to 802.11 decoder.
Change decoding and extraction errors to warnings.

When a writable stream emits an 'error' event, all readable streams
writing to it are unpiped. In our use-case, it becomes prohibitively
expensive to reattach them (especially since errors are out of our
control because of packet corruption), which motivates the use of a
different event: 'warning'.
Improve 802.11 decoder further.