Make ICMPv6 hard error handling for TCP consistent with the ICMPv4

Make ICMPv6 hard error handling for TCP consistent with the ICMPv4
handling. Ensure that:
* Protocol unreachable errors are handled by indicating ECONNREFUSED
  to the TCP user for both IPv4 and IPv6. These were ignored for IPv6.
* Communication prohibited errors are handled by indicating ECONNREFUSED
  to the TCP user for both IPv4 and IPv6. These were ignored for IPv6.
* Hop Limited exceeded errors are handled by indicating EHOSTUNREACH
  to the TCP user for both IPv4 and IPv6.
  For IPv6 the TCP connected was dropped but errno wasn't set.

Reviewed by: gallatin, rrs
MFC after: 1 month
Sponsored by: Netflix
Differential Revision: 7904