[reSIProcate] isReliable() and isSecure()

[Daniel Pocock <daniel@xxxxxxxxxxxxx>]

I notice quite a few places where we test transport types using
Symbols::TLS and/or TransportType TLS and various combinations of TCP or
DTLS, etc

I've added a method isSecure(), much like isReliable(), and changed many
other pieces of code to use those two methods where I think it is
appropriate.

I wasn't quite sure about those places that refer to SigComp: does
SigComp only make sense on TCP || TLS?  Or could it be valid with
WS/WSS?  If so, the sigcomp related transport checks can also use
isReliable()