openssl: set FLAG_TRUSTED_FIRST unconditionally

[freedge]

On some systems, openssl 1.0 is still the default, but it has been patched to
contain all the recent security fixes. As a result of this patching, it is
possible for macro X509_V_FLAG_NO_ALT_CHAINS to be defined, while the previous
behavior of openssl to not look at trusted chains first, remains.

Problem is that curl will fail to verify websites stills serving an expired, intermediate certificate, which happened quite a few times recently: https://www.agwa.name/blog/post/fixing_the_addtrust_root_expiration

Fix it: ensure X509_V_FLAG_TRUSTED_FIRST is always set, do not try to probe for
the behavior of openssl based on the existence of this macro.

[bagder]

Thanks!

[jay]

Too fast did you investigate his claim? I was the one who set it not to use trusted first when alt chains was in effect. @mattcaswell wrote "It makes no sense to combine the trusted first and alt chains strategies. With trusted first we have already checked all of the possible chains by the time we get to the end of the peer provided list - so there is no point in then popping certs off the top of our chain and checking the trusted store again."

Ref: https://curl.haxx.se/mail/lib-2020-06/0001.html
Ref: https://curl.haxx.se/mail/archive-2020-06/0011.html

[bagder]

1. From what I've been told this is what OpenSSL >= 1.1.0 does by itself already
2. This is what python does and thanks to this they avoided the AddTrust issue
3. This actually fixes works-around the AddTrust issue

So, no, it wasn't too fast. But I'm open for alternative takes. Not sure spending a lot of brain-cells on old OpenSSL versions is worth it though.