Split sifting logic into separate function

[mithrandi]

certificateOptionsFromPEMs actually does two things at the moment:

1. Sifts a pile of PEM objects into key + cert + chain certs (and with Support loading DH parameters in pem.twisted.certificateOptionsFromFiles #18, dhparams).
2. Turns those things into a CertificateOptions.

I'd kinda like to have 1 without 2, since CertificateOptions has OpenSSL.crypto.x509 objects instead of cryptography.x509 objects, the latter having a much more useful API. @hynek do you think it's sensible to split these up?

[hynek]

Yes, because pyOpenSSL will hopefully move to cryptography.x509 too pyca/pyopenssl#439 .

I’m also idling thinking about supporting stdlib by writing those files into temporary files and load them from there.

[mithrandi]

Gah. This can't work exactly as stated, because there's no way to know which cert goes with the private key until you've loaded them with something that can give you the public key...