Add a check for missing environment variable documentation

[esyr]

Based on [1], it seems prudent to have a check (that is run as part of check_docs CI test) that verifies that all environment variables used by OpenSSL libraries and commands are documented. Seems like util/find-doc-nits is the best place for it, but not necessarily.

[1] #28025 (comment)

[esyr]

A couple of side notes:

• not all envvars can be easily obtained via source-level analysis: for example, SSL_CERT_DIR comes from X509_get_default_cert_dir_env() function, that returns it;
• not only *getenv() functions are to be considered: for Windows, it's GetEnvironmentVariableW(), and some Perl scripts that are shipped as part of the distribution, there's the ENV array (that's where OPENSSL, OPENSSL_CONFIG, and TSGET come from);
• some variables are already listed in util/other.syms, maybe it's the way to deal with the first point, but it's definitely not future-proof, as this list doesn't seem to be consistently updated.

[paulidale]

• some of the getenv() calls don't take a constant string: ossl_safe_getenv(use_ssl ? "https_proxy" : "http_proxy");
• some are via macros: ossl_safe_getenv(use_ssl ? OPENSSL_HTTPS_PROXY : OPENSSL_HTTP_PROXY);
• some are going to be tricky to figure out: ssl_safe_getenv(name);

and likely a number of other painful cases. Nonetheless, a script that covers the majority of cases should be reasonable.

[jogme]

@paulidale I have a draft PR, but it only takes care of the const string names and none of the cases mentioned by you. Also I don't know how to easily check for them. Do you have a solution on mind or idea how to do it?

[paulidale]

I don't have a solution in mind. I manually checked these since there weren't many instances. The ones using ?: and macros ought to be possible to automate.