Handle the link-layer header more like other 802.x dissectors.

Handle the link-layer header more like other 802.x dissectors.

Have ieee_802_11_hdr_print() unconditionally print the MAC header, call
it before any payload dissection if -e is specified, call it if
llc_print() fails and -e *isn't* specified, and print "WEP Encrypted"
for protected management and control frames if -e isn't specified.

Pull the extraction of source and destination addresses out of
ieee_802_11_hdr_print(), and do it only for data frames, as it's not
needed for management or control frames.