mk-ca-bundle.vbs: Fix UTF-8 output

- Change initial message box to mention delay when downloading/parsing.

Since there is no progress meter it was somewhat unexpected that after
choosing a filename nothing appears to happen, when actually the cert
data is in the process of being downloaded and parsed.

- Warn if OpenSSL is not present.

- Use a UTF-8 stream to make the ca-bundle data.

- Save the UTF-8 ca-bundle stream as binary so that no BOM is added.

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This is a follow-up to d2c6d156 which switched mk-ca-bundle.vbs output to
ANSI due to corrupt UTF-8 output, now fixed.

This change completes making the default certificate bundle output of
mk-ca-bundle.vbs as close as possible to that of mk-ca-bundle.pl, which
should make it easier to review any difference between their output.

Ref: https://github.com/curl/curl/pull/1012