The new name readCipherPacket/writeCipherPacket disambiguates method
calls from packetConn.{read,write}Packet, which has a different
signature, and is called in different layers of the stack.
Change-Id: I02ca83e28f74f0a347a1cde665efdfb78f3f51bb
Reviewed-on: https://go-review.googlesource.com/c/crypto/+/171657
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>