If the --key-file output filename ends in ".pfx" or ".p12", use pkcs12
format.
This might not be working entirely correctly yet but might be enough for
others to help out or experiment.
Updates #2928
Updates #5011
Change-Id: I62eb0eeaa293b9fd5e27b97b9bc476c23dd27cf6
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>