Check for TLV length too small.

The TLV length includes the T and the L, so it must be at least 4.

This means we don't need the "avoid infinite loop" check later; that
check was wrong, as per GitHub issue #401 and #402; this fixes #402,
which has a different patch for that bug.
Merge git://github.com/the-tcpdump-group/tcpdump
Qualify "length" when printing it.

In the "the TLV length is too short" message, we're printing the length
of the entire TLV; report it as "TLV length".  If we pass that test,
we've subtracted out the lengths of the T and the L, leaving only the
length of the V, so report it as "value length".
Update to reflect the output format changes and bug fixes for CDP.