crypto/tls: generate random serial numbers.
crypto/tls: generate random serial numbers.

NSS (used in Firefox and Chrome) won't accept two certificates with the same
issuer and serial. But this causes problems with self-signed certificates
with a fixed serial number.

This change randomises the serial numbers in the certificates generated by
generate_cert.go.

R=golang-dev, r
CC=golang-dev
https://codereview.appspot.com/38290044

h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang