netchan: rather than 0, make -1 mean infinite, by analogy with strings.Split etc.

It's unlikely to affect any extant code, but I wanted to make this API change
before digging in for a rewrite.

R=rsc
CC=golang-dev
https://golang.org/cl/2112041

src/pkg/netchan/export.go

@@ -154,8 +154,9 @@ func (client *expClient) serveRecv(hdr header, count int) {
 			client.sendError(&hdr, err.String())
 			break
 		}
-		if count > 0 {
-			if count--; count == 0 {
+		// Negative count means run forever.
+		if count >= 0 {
+			if count--; count <= 0 {
 				break
 			}
 		}

src/pkg/netchan/import.go

@@ -114,14 +114,14 @@ func (imp *Importer) run() {
 }
 
 // Import imports a channel of the given type and specified direction.
-// It is equivalent to ImportNValues with a count of 0, meaning unbounded.
+// It is equivalent to ImportNValues with a count of -1, meaning unbounded.
 func (imp *Importer) Import(name string, chT interface{}, dir Dir) os.Error {
 	return imp.ImportNValues(name, chT, dir, 0)
 }
 
 // ImportNValues imports a channel of the given type and specified direction
 // and then receives or transmits up to n values on that channel.  A value of
-// n==0 implies an unbounded number of values.  The channel to be bound to
+// n==-1 implies an unbounded number of values.  The channel to be bound to
 // the remote site's channel is provided in the call and may be of arbitrary
 // channel type.
 // Despite the literal signature, the effective signature is