The pointer to std::vector is unnecessary, so replace it with standard
instance. Also, make the for() loop more readable by using actual type
instead of inferred - there is no readability benefit here from
obfuscating the type.

NodeJS can already use an external, shared OpenSSL library. This
library knows where to look for OS managed certificates. Allow
a compile-time option to use this CA store by default instead of
using bundled certificates.

In case when using bundled OpenSSL, the paths are also valid for
majority of Linux systems without additional intervention. If
this is not set, we can use SSL_CERT_DIR to point it to correct
location.

Fixes: nodejs#3159
PR-URL: nodejs#8334