net: check /etc/hosts for modifications every 5 seconds, not 5 minutes

But also cache the previous parsed form and don't reread if the
size and modification time are both unchanged from before.

On systems with stable /etc/hosts this should result in more stat calls
but only a single parsing of /etc/hosts.

On systems with variable /etc/hosts files (like some Docker systems)
this should result in quicker adoption of changes.

Fixes #13340.

Change-Id: Iba93b204be73d6d903cd17c58038a4fcfd0952b9
Reviewed-on: https://go-review.googlesource.com/18258
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/net/parse.go

@@ -10,6 +10,7 @@ package net
 import (
 	"io"
 	"os"
+	"time"
 	_ "unsafe" // For go:linkname
 )
 
@@ -71,6 +72,14 @@ func open(name string) (*file, error) {
 	return &file{fd, make([]byte, 0, os.Getpagesize()), false}, nil
 }
 
+func stat(name string) (mtime time.Time, size int64, err error) {
+	st, err := os.Stat(name)
+	if err != nil {
+		return time.Time{}, 0, err
+	}
+	return st.ModTime(), st.Size(), nil
+}
+
 // byteIndex is strings.IndexByte. It returns the index of the
 // first instance of c in s, or -1 if c is not present in s.
 // strings.IndexByte is implemented in  runtime/asm_$GOARCH.s