Add back `SystemCertPool()` to provide an OS-agnostic way to get the root certs

[phillip-stephens]

Not sure if this is an issue, since it seems it was very clearly removed on purpose. However, I'm unsure of why it's a good idea to remove this functionality.

This ZGrab issue discusses how without a way to automatically find the root store, users have to specify it. IMO, this raises a hurdle to using ZGrab with TLS unnecessarily.

SystemCertPool() was removed in 0b16e8f The commit msg specifies that zmap/rootfetch should now be used.

Golang crypto/x509 SystemCertPool() Docs

@dadrian or @zakird, do you have more context on this? This seems like a really useful thing to have been removed from zcrypto, unless I'm missing some motivation.

[dadrian]

I don't know if this is still the case, but at the time:

• SystemCertPool required cgo and caused a bunch of compile errors on platforms we didn't have CI for. It also prevented us from easily doing builds for Linux from Mac.
• In general, we didn't want to use the various root stores and instead wanted to provide our own, so it stopped us from accidentally shooting ourselves in the foot.
• IIRC, it also had some thread safety issues for our use case, but I don't remember, it's been a decade