Support env vars SSL_CERT_FILE and SSL_CERT_DIR on linux

[jonasfj]

As of https://github.com/dart-lang/sdk/commit/139db22be5f515b49270f6eae80ff7ecbfab1620 the Dart SDK certificate store being specified by --root-certs-file= and --root-certs-cache=. But this is not very convenient, perhaps we should consider supporting:

• $SSL_CERT_FILE
• $SSL_CERT_DIR (may contain multiple directories separated :)

Like golang/src/crypto/x509/root_unix.go.

Also it seems golang:

• Reads from the first bundle file found, AND,
• Reads from all directories found.

Where as dart, seems to:

• Read from the first bundle file found, OR
• Read from the directory found (dart only checks a single directory).

I don't know if that's a mistake, or intentional, or just me reading the golang code wrong :)

Also golang/src/crypto/x509/root_linux.go searches a lot more folders and files than the Dart SDK does.

[a-siva]

//cc @brianquinlan