[SIP] Adding check for SIP/2.0 for packets recieved at SIP port

Packets arriving at SIP port will now be checked for SIP/2.0 before
getting tagged as SIP packet. Added helper functions strnstr and strnstrn
for checking substring in a string in case of non null trmination.

Acked-by: Sudhanshu Singh <sudhsing@codeaurora.org>
Signed-off-by: Mohammed Javid <mjavid@codeaurora.org>
Change-Id: Ia7d6a3ac5dd1e5f0ce97ee9f6d0c13f16d9dd7f2