Add native-certs support

[kpcyrd]

I've packaged the irc crate for Debian yesterday, and the policy forbids use of the webpki-roots crate, because it duplicates the official certificate store. I patched it to rustls-native-certs, this adds some of the code upstream so I only need to remove the "webpki-roots" crate from Cargo.toml with a downstream patch and everything else "falls into place". :)

It also adds support for rustls-native-certs, but won't cause a failure if it errors.