syscall: extract an ExampleLoadLibrary from comment

while we are at it, fix some out-of-date comments.

R=golang-dev, dave, r
CC=golang-dev
https://golang.org/cl/6498054

src/pkg/syscall/syscall_windows_test.go

@@ -49,3 +49,24 @@ func TestWin32finddata(t *testing.T) {
 		t.Fatalf("memory corruption: want=%d got=%d", want, x.got)
 	}
 }
+
+func abort(funcname string, err error) {
+	panic(funcname + " failed: " + err.Error())
+}
+
+func ExampleLoadLibrary() {
+	h, err := syscall.LoadLibrary("kernel32.dll")
+	if err != nil {
+		abort("LoadLibrary", err)
+	}
+	defer syscall.FreeLibrary(h)
+	proc, err := syscall.GetProcAddress(h, "GetVersion")
+	if err != nil {
+		abort("GetProcAddress", err)
+	}
+	r, _, _ := syscall.Syscall(uintptr(proc), 0, 0, 0, 0)
+	major := byte(r)
+	minor := uint8(r >> 8)
+	build := uint16(r >> 16)
+	print("windows version ", major, ".", minor, " (Build ", build, ")\n")
+}