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/ftp.c

@@ -2456,7 +2456,7 @@ static CURLcode ftp_statemach_act(struct connectdata *conn)
         default:
           failf(data, "unsupported parameter to CURLOPT_FTPSSLAUTH: %d",
                 (int)data->set.ftpsslauth);
-          return CURLE_FAILED_INIT; /* we don't know what to do */
+          return CURLE_UNKNOWN_OPTION; /* we don't know what to do */
         }
         PPSENDF(&ftpc->pp, "AUTH %s", ftpauth[ftpc->count1]);
         state(conn, FTP_AUTH);