This CL reduces unnecessary allocations in ancillary helper
functions. Also clarifies documentation on ControlMessage,
ControlFlags.

benchmark                     old allocs   new allocs    delta
BenchmarkReadWriteNetUDP-2             6            6    0.00%
BenchmarkReadWriteIPv4UDP-2           17           15  -11.76%

R=dave
CC=golang-dev
https://golang.org/cl/9232050