Ability to Parse bitmasked invite requests

Hey Guys,

We tweaked kamailio a little so we could parse our own invite requests. We mask the invite requests with 4 bits at the start of the message so we can avoid sip-inspect on some routers from stealing our requests and re-writing them, this code will ensure those requests are caught and handled correctly by kamailio.

We do this to avoid unexpected behaviors by some routers poorer implementations of sip-inspect, this allows us to facilitate more p2p sessions with our client side code.

I'd love if you guys could include this in your package (It will save us from patching each release). But I understand if you deem this out of scope for this as it really only effects our own projects.