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