Rewrite leftover references to plan9.bell-labs.com to 9p.io.

Change-Id: Iadb4aa016a7b361d01827787dbc59164d5d147f2
Reviewed-on: https://go-review.googlesource.com/20291
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/cmd/yacc/doc.go

@@ -15,7 +15,7 @@ It is largely transliterated from the Inferno version written in Limbo
 which in turn was largely transliterated from the Plan 9 version
 written in C and documented at
 
-	http://plan9.bell-labs.com/magic/man2html/1/yacc
+	https://9p.io/magic/man2html/1/yacc
 
 Adepts of the original yacc will have no trouble adapting to this
 form of the tool.

src/debug/gosym/symtab.go

@@ -8,7 +8,7 @@
 package gosym
 
 // The table format is a variant of the format used in Plan 9's a.out
-// format, documented at http://plan9.bell-labs.com/magic/man2html/6/a.out.
+// format, documented at https://9p.io/magic/man2html/6/a.out.
 // The best reference for the differences between the Plan 9 format
 // and the Go format is the runtime source, specifically ../../runtime/symtab.c.
 

src/image/color/palette/gen.go

@@ -89,7 +89,7 @@ func printPlan9(w io.Writer) {
 	fmt.Fprintln(w, "// of continuous tones.")
 	fmt.Fprintln(w, "//")
 	fmt.Fprintln(w, "// This palette was used in the Plan 9 Operating System, described at")
-	fmt.Fprintln(w, "// http://plan9.bell-labs.com/magic/man2html/6/color")
+	fmt.Fprintln(w, "// https://9p.io/magic/man2html/6/color")
 	fmt.Fprintln(w, "var Plan9 = []color.Color{")
 	for _, line := range lines {
 		fmt.Fprintln(w, line)

src/image/color/palette/palette.go

@@ -19,7 +19,7 @@ import "image/color"
 // of continuous tones.
 //
 // This palette was used in the Plan 9 Operating System, described at
-// http://plan9.bell-labs.com/magic/man2html/6/color
+// https://9p.io/magic/man2html/6/color
 var Plan9 = []color.Color{
 	color.RGBA{0x00, 0x00, 0x00, 0xff},
 	color.RGBA{0x00, 0x00, 0x44, 0xff},

src/path/filepath/path.go

@@ -80,7 +80,7 @@ const (
 //
 // See also Rob Pike, ``Lexical File Names in Plan 9 or
 // Getting Dot-Dot Right,''
-// http://plan9.bell-labs.com/sys/doc/lexnames.html
+// https://9p.io/sys/doc/lexnames.html
 func Clean(path string) string {
 	originalPath := path
 	volLen := volumeNameLen(path)

src/path/path.go

@@ -65,7 +65,7 @@ func (b *lazybuf) string() string {
 //
 // See also Rob Pike, ``Lexical File Names in Plan 9 or
 // Getting Dot-Dot Right,''
-// http://plan9.bell-labs.com/sys/doc/lexnames.html
+// https://9p.io/sys/doc/lexnames.html
 func Clean(path string) string {
 	if path == "" {
 		return "."