When parsing IEs in a management frame, save the *first* IE of a given

When parsing IEs in a management frame, save the *first* IE of a given
type we see, not the *last*, so the first one gets printed.  If,
however, a rates IE has a zero-length list of rates, don't save that
one; some devices send out frames with two rates IEs, one zero-length
and a later one containing the actual rates.

Have parse_elements() return an indication of whether truncation
occurred - and pass it the frame length, so it doesn't just keep parsing
until an end-of-captured-data test fails, as that would make *every*
frame look truncated.