* new classe p3zcNatAssist
* moved some common ZeroConf functions / definitions around.
* moved virtual tick() to parent class: pqiNetAssist
* added zcNetAssist section to makefiles. (active for OSX build)
* setup switch between p3zcNatAssist & upnphandler in rsinit.cc
NOTE: Still to test network restart, etc, and correct exchange of external Ip Address.
git-svn-id: http://svn.code.sf.net/p/retroshare/code/trunk@4800 b45a01b8-16f6-495d-af2f-9b41ad6348cc