curl: A logic error in detect_proxy caused truncation of environment variable names for long protocol schemes.

In lib/url.c, the detectproxy function uses a fixed-size buffer, proxyenv20, to construct proxy environment variable names e.g., httpproxy. However, the curl URL parser lib/urlapi.c allows protocol schemes up to 40 characters MAXSCHEMELEN. When a protocol scheme longer than 12 characters is used,...