return code cleanup: build, init and run-time errors

Stop the abuse of CURLE_FAILED_INIT as return code for things not being
init related by introducing two new return codes:

CURLE_NOT_BUILT_IN and CURLE_UNKNOWN_OPTION

CURLE_NOT_BUILT_IN replaces return code 4 that has been obsoleted for
several years. It is used for returning error when something is
attempted to be used but the feature/option was not enabled or
explictitly disabled at build-time. Getting this error mostly means that
libcurl needs to be rebuilt.

CURLE_FAILED_INIT is now saved and used strictly for init
failures. Getting this problem means something went seriously wrong,
like a resource shortage or similar.

CURLE_UNKNOWN_OPTION is the option formerly known as
CURLE_UNKNOWN_TELNET_OPTION (and the old name is still present,
separately defined to be removed in a very distant future). This error
code is meant to be used to return when an option is given to libcurl
that isn't known. This problem would mostly indicate a problem in the
program that uses libcurl.

lib/http_proxy.h

@@ -29,5 +29,5 @@ CURLcode Curl_proxyCONNECT(struct connectdata *conn,
 #define PROXY_TIMEOUT (3600*1000)
 
 #else
-#define Curl_proxyCONNECT(x,y,x,w) CURLE_FAILED_INIT
+#define Curl_proxyCONNECT(x,y,x,w) CURLE_NOT_BUILT_IN
 #endif