net/http: increase panic stack trace buffer size from 4 KB to 64 KB.

4 KB is a bit too small in some situations (e.g. panic during a
template execution), and ends up with an unhelpfully-truncated trace.
64 KB should be much more likely to capture the useful information.
There's not a garbage generation issue, since this code should only
be triggered when there's something seriously wrong with the program.

LGTM=bradfitz
R=bradfitz
CC=golang-codereviews
https://golang.org/cl/63520043

src/pkg/net/http/server.go

@@ -1083,7 +1083,7 @@ func validNPN(proto string) bool {
 func (c *conn) serve() {
 	defer func() {
 		if err := recover(); err != nil {
-			const size = 4096
+			const size = 64 << 10
 			buf := make([]byte, size)
 			buf = buf[:runtime.Stack(buf, false)]
 			log.Printf("http: panic serving %v: %v\n%s", c.remoteAddr, err, buf)