Improve handling of key environment variables (as they do not work on Windows systems)

[jmckenna]

• this is a long reported issue (many tickets in MapServer codebase over a decade or more) where environment variables do not work on all Windows systems (no matter what the web server is)
• an example key environment variable, that is relied upon by the MapCache codebase is CURL_CA_BUNDLE
• MapCache in /mapcache/lib/http.c faces this same problem on Windows systems (MS4W has had to do various workarounds because of this, for years, for both MapCache and MapServer)
• pull request https://github.com/MapServer/MapServer/pull/6304 enables use of that variable through the mapfile CONFIG parameter
• update: I wonder if we could have a new section in mapcache.xml named <environment_variables>, and these would take precedence (use this value, otherwise use the one found at the system level).

Thoughts?

[geographika]

@jmckenna - where wre you able to pass the file path in CURL_CA_BUNDLE to cURL? In MapServer we have the following line at https://github.com/MapServer/MapServer/blob/c862b04cf690091995df40139a6be77a2ff72bce/maphttp.c#L579

      unchecked_curl_easy_setopt(http_handle, CURLOPT_CAINFO, pszCurlCABundle );

The XML approach sounds good. Alternatively the CPLGetConfigOption approach used in MapServer could be used.