This change include updates to the probeIPv4Stack
and probeIPv6Stack to ensure that one or both
protocols are supported by ip(3).
The addition of fdMutex to netFD fixes the
TestTCPConcurrentAccept failures.
Additional changes add support for keepalive.

R=golang-codereviews, 0intro
CC=golang-codereviews, rsc
https://golang.org/cl/49920048