chat: Remove variable name conflicts from VDL generated Go code

The previous VDL generated Go code contained common variable
names, such as ctx, opts and err.  This meant that users couldn't
pick these names for their variables.  This CL fixes those cases.

Note that we already generate different code for exported
interfaces vs. unexported implementations.  The interfaces use
user-provided variable names, which is better for documentation,
while the implementations already avoid conflicts by using
generated variables names i0, i1, ... for input args, and
similarly o0, o1, ... for output args.

This CL ensures the generated interfaces use _ for non-user
provided variables, to avoid any possible naming conflict.

Fixes v.io/i/912

MultiPart: 5/7

Change-Id: I4be563e3a564b2eb7879eb146337a94365fe92d5
diff --git a/clients/shell/go/src/v.io/x/chat/vdl/chat.vdl.go b/clients/shell/go/src/v.io/x/chat/vdl/chat.vdl.go
index 8f80e1b..c77e629 100644
--- a/clients/shell/go/src/v.io/x/chat/vdl/chat.vdl.go
+++ b/clients/shell/go/src/v.io/x/chat/vdl/chat.vdl.go
@@ -18,7 +18,7 @@
 // containing Chat methods.
 type ChatClientMethods interface {
 	// SendMessage sends a message to a user.
-	SendMessage(ctx *context.T, text string, opts ...rpc.CallOpt) error
+	SendMessage(_ *context.T, text string, _ ...rpc.CallOpt) error
 }
 
 // ChatClientStub adds universal methods to ChatClientMethods.
@@ -45,7 +45,7 @@
 // implements for Chat.
 type ChatServerMethods interface {
 	// SendMessage sends a message to a user.
-	SendMessage(ctx *context.T, call rpc.ServerCall, text string) error
+	SendMessage(_ *context.T, _ rpc.ServerCall, text string) error
 }
 
 // ChatServerStubMethods is the server interface containing