Suppress OpenSSL error on missing optional config

openssl_pkey_new() fetches various options from the config file --
most of these are optional, and not specifying them is not an error
condition from the perspective of the user. Unfortunately, the
CONF_get_string() API pushes an error when accessing a key that
doesn't exist (_CONF_get_string does not, but that is presumably a
private API). This commit adds a helper php_openssl_conf_get_string()
that automatically clears the error in this case. I've found that
OpenSSL occasionally does the same thing internally:
https://github.com/openssl/openssl/blob/22040fb790c854cefb04bed98ed38ea6357daf83/apps/req.c#L515-L517

Closes GH-6699.

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