syscall: fix infinite recursion in itoa

Fixes issue 8332.

LGTM=dvyukov
R=golang-codereviews, dvyukov
CC=golang-codereviews
https://codereview.appspot.com/138650044
bytes, strings: document that FieldsFunc f must not be stateful

Fixes issue 8738.

LGTM=adg
R=golang-codereviews, adg
CC=golang-codereviews
https://codereview.appspot.com/143260045