crypto/x509: load certs from env vars + extra locations
Add the ability to override the default file and directory from
which certificates are loaded by setting the OpenSSL compatible
environment variables: SSL_CERT_FILE, SSL_CERT_DIR.
If the variables are set the default locations are not checked.
Added new default file "/usr/local/etc/ssl/cert.pem" for FreeBSD.
Certificates in the first valid location found for both file and
directory are added, instead of only the first file location if
a valid one was found, which is consistent with OpenSSL.
Fixes #3905
Fixes #14022
Fixes #14311
Fixes #16920
Fixes #18813 - If user sets SSL_CERT_FILE.