Parse an application/x-x509-user-cert response with

[This CL is for reference only.]

Parse an application/x-x509-user-cert response with
net::X509Certificate::CreateCertificateListFromBytes(), which supports
three formats (in particular PKCS #7).

Save the response in a file named "keygen-response".  Print messages
about the format and contents of the response.

Import the intermediate CA certificates in the response (only implemented
for NSS).  NOTE: we must verify the CA certificates before importing them.

Sign PublicKeyAndChallenge with sha1WithRSAEncryption.

R=rsleevi@chromium.org,mattm@chromium.org
BUG=37142
TEST=none