SSL_CERT_{DIR, FILE} handling is incorrect

The way these are used is that *both* variables apply and add root certificates. Moreover, these variables are delimiter-separated lists, so the current API convention of checking `isdir` on the result to determine whether to treat it as a file or directory is broken. My recommendation would be to add a separate API function `ca_root_locations()::Union{Nothing, Tuple{Vector{String}, Vector{String}}}` that gives a list of both files and directories to search and have the current `ca_roots` API fall back to picking the first of these (prioritizing files over directories) for backwards compatibility.