A FlagSet is an independent set of flags that may be used,
for example, to provide flag processing for subcommands
in a CLI. The standard, os.Args-derived set of flags is a
global but non-exported FlagSet and the standard functions
are wrappers for methods of that FlagSet.
Allow the programmer to control whether the program
exits if there is a parse error. For the default set, the behavior
remains to exit on error.
The handling of Usage is odd due to backward compatibility.
R=golang-dev, bradfitz, r, bradfitz
CC=golang-dev
https://golang.org/cl/4517092
Rob Pike
Russ Cox