Adds readPacket() to conn, and renames conn to packetConn. Key exchanges operate on packetConn, so they can be unittested. R=agl, jpsugar, dave CC=golang-dev https://golang.org/cl/13352055
R=golang-dev, mikioh.mikioh, remyoudompheng CC=golang-dev https://golang.org/cl/13370043
Implement elliptic-curve Diffie-Hellman, including host key signature verification. Moves host key cryptographic verification to ClientConn.handshake(), so RSA host keys are also verified. Fixes golang/go#6158. R=dave, agl CC=golang-dev https://golang.org/cl/13021045