flag: disallow setting flags multiple times

flag: disallow setting flags multiple times
This is a day 1 error in the flag package: It did not check
that a flag was set at most once on the command line.
Because user-defined flags may have more general
properties, the check applies only to the standard flag
types in this package: bool, string, etc.

Fixes #8960.

LGTM=bradfitz
R=golang-codereviews, bradfitz
CC=golang-codereviews
https://golang.org/cl/156390043