[reSIProcate] Const-correctness

[Kenneth Ho]

Guys,

I fixed tons of const-correctness on a branch, including buildFdSet()s, 
getTimeTillNextProcessMs()s, related functions and etc. Details in rev 
5648 commit log.

People who has time please look into diffs and verify if they are fine. 
I am not sure if I got all related functions, since I've only compiled 
resip/dum with my local options. If someone would like to help, please 
do so by merging the mods back to trunk once they are verified, or I 
would do it at a later time.

Please note that I've already merged a part of rev 5648 to trunk, 
detilas in rev 5650 commit log.

Ken