Fix for bug 1279 (Negative values for RTCP round trip delay

Fix for bug 1279 (Negative values for RTCP round trip delay
cannot be stored in guint32).
- Makes the threshold preference value an absolute value.
- There is now a separate expert info item for -ve roundtrips

N.B. There is still a problem with filtering -ve values on this
FT_INT32 field, i.e. rtcp.roundtrip-delay < 0 never matches with
frames that it should (even if rtcp.roundtrip-delay == -3 can
be used to match specific frames...).

git-svn-id: http://anonsvn.wireshark.org/wireshark/trunk@20264 f5534014-38df-0310-8fa8-9805f1628bb7