In resolving 3507, the fix had to be applied individually to
the four *Conn types, tcp, udp, rawip and unix, due to the
duplicate code in each Conn type.

This CL consolidates the common net.Conn methods that all four
*Conn types implement into a base conn type.

Pros:
* The fix for 3507 would have only needed to be applied to one
method. Further improvements, such as possibly removing the
c.fd != nil check in c.ok(), would benefit from this CL.
* Nearly 300 lines removed from the net package.
* The public interface and documentation are not changed.
* I think this is an excellent example of the power of embedding.

Cons:
* The net package is already distributed over many files, this
CL adds another place to look.
* The fix for 3507 was a total of 16 lines changed, this follow
up CL could be considered to be an overreaction as new Conn types
are unlikely to be added in the near future.

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/6098047