net: introduce net.Error interface

net: introduce net.Error interface

Adds two more methods, Timeout and Temporary.
Implemented by os.Errno too.  The intent is to make
the checks for os.EAGAIN a little less clunky.
It should also let us clean up a bug that Mike Solomon
pointed out: if a network server gets an "out of file descriptors"
error from Accept, the listener should not stop.
It will be able to check this because that error would
have Temporary() == true.

Also clean up some underscore names.

Fixes #442.

R=r
CC=golang-dev, msolo
https://golang.org/cl/957045