ipc: VCSecurityLevel->Security and no longer a stream opt.

SecurityLevel is not handled only in server.go and client.go.
Everywhere else in the ipc stack, nil principal means SecurityNone.

This also fixes https://github.com/veyron/release-issues/issues/1423
in the process.

MultiPart: 1/2
Change-Id: I55646e574e43b633c69300b1fecb623c95340bbc
diff --git a/security/agent/server/wire.vdl b/security/agent/server/wire.vdl
index 9202cd7..53d94d4 100644
--- a/security/agent/server/wire.vdl
+++ b/security/agent/server/wire.vdl
@@ -7,7 +7,7 @@
 // unix domain socket. To connect to the agent, a client should create
 // a unix domain socket pair. Then send one end of the socket to the agent
 // with 1 byte of data. The agent will then serve the Agent service on
-// the recieved socket, using VCSecurityNone.
+// the received socket, using SecurityNone.
 //
 // The agent also supports an optional mode where it can manage multiple principals.
 // Typically this is only used by Device Manager. In this mode, VEYRON_AGENT_FD