cmd/stringer: move away from using os.Args[0]

stringer's usage currently uses os.Args[0] to refer to the program name
as a defensive measure in case stringer is compiled and then run using a
name other than stringer, e.g. mystringer. However, the following lines
of the usage docs hardcode "stringer." So it seems this has little
benefit.

Indeed this style of using os.Args[0] can make the usage information of
stringer extremely ugly when stringer itself is invoked with an absolute
path:

Usage of /tmp/tmp.ahddh3ZjE7/.gobincache/golang.org/x/tools/@v/v0.0.0-20181030000716-a0a13e073c7b/golang.org/x/tools/cmd/stringer/stringer:
        stringer [flags] -type T [directory]
        stringer [flags] -type T files... # Must be a single package

...

Instead, opt for the simpler approach of hardcoding stringer throughout.

Change-Id: Ia296c8d9a91c94e77b7cc2b6242908c959fb5985
Reviewed-on: https://go-review.googlesource.com/c/145799
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
Reviewed-by: Alan Donovan <adonovan@google.com>
Reviewed-by: Rob Pike <r@golang.org>

cmd/stringer/stringer.go

@@ -87,7 +87,7 @@ var (
 
 // Usage is a replacement usage function for the flags package.
 func Usage() {
-	fmt.Fprintf(os.Stderr, "Usage of %s:\n", os.Args[0])
+	fmt.Fprintf(os.Stderr, "Usage of stringer:\n")
 	fmt.Fprintf(os.Stderr, "\tstringer [flags] -type T [directory]\n")
 	fmt.Fprintf(os.Stderr, "\tstringer [flags] -type T files... # Must be a single package\n")
 	fmt.Fprintf(os.Stderr, "For more information, see:\n")