Added a native ICE client, so P2P can be done without WebRTC. This client is beta and is not enabled by default, unless WebRTC support is disabled. The client does not currently support TURN.

Improvements and bugfixes:

• Fix protocol bug in malicious sender protection, that could cause asserts and possibly dropped reliable messages if packet loss and latency were both extremely high.
• Polling will now use epoll on supported platforms (e.g. linux).
• IPv6 parsing now supports dotted-decimal-style IPv4-mapped IPv6 addresses, e.g. ::ffff:123.45.67.89
• Fix deadlock if API calls were made from custom P2P callbacks that needed to lookup a connection by handle
• Public headers no longer #define POSIX, which conflicts with other libraries
• Improvements to platform detection and compatibility on various console platforms

No public interfaces were changed