krb5-gssapi: Delete the GSS-API context.

This fixes a memory leak related to the GSS-API code.

Added a krb5_init and krb5_end functions. Also removed a work-around
the lack of proper initialization of the GSS-API context.

lib/ftp.c

@@ -3860,6 +3860,10 @@ static CURLcode ftp_disconnect(struct connectdata *conn)
 
   Curl_pp_disconnect(pp);
 
+#if defined(HAVE_KRB4) || defined(HAVE_GSSAPI)
+  Curl_sec_end(conn);
+#endif
+
   return CURLE_OK;
 }