[SSL] Add ability to set verification flags

[Bonias]

Description

Add ability to set OpenSSL verification flags (https://www.openssl.org/docs/manmaster/man3/X509_VERIFY_PARAM_set_hostflags.html#VERIFICATION-FLAGS).

We had a need to set X509_V_FLAG_PARTIAL_CHAIN flag. Most major TLS libraries behaves like this by default but not OpenSSL (openssl/openssl#7871). Adding ability to set any verification flag allows us to set X509_V_FLAG_PARTIAL_CHAIN flag. I think it may be useful for others too.

I implemented this for MRI only as I don't know how to do this for JRuby (and if this is even required?).

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[MSP-Greg]

@Bonias

Not that familiar with these flags, or how and when they may be set. In mini_ssl.c, might it be better to change:

    param = X509_VERIFY_PARAM_new();

to something like:

    param = SSL_CTX_get0_param(ctx);

[Bonias]

Thanks @MSP-Greg! I changed it to SSL_CTX_get0_param

[MSP-Greg]

@Bonias

Well, I found out what one can't do with git...

I rebased this, adjusted dsl.rb, then hit a compile issue, fixed that, then decided that, rather than repeating &verification_flags= in the query string, that concatenating the strings would be a lot easier to look at...

I don't know what you think, but the patch is at:

https://github.com/MSP-Greg/puma/commit/4d063d982c5.patch

If you rebase, then replace the PR commit with the above, it should be good to merge, and CI did pass...

[Bonias]

I rebased this. I also updated README and History files. I hope all is good now.

[MSP-Greg]

@Bonias

Thank you. Sorry for the trouble, the changes I made to DSL got merged first, so this got messy.

The DSL changes allow access to the ssl bind string, which makes CI a bit easier...