failf() now only overwrites the error buffer the first time it gets called

for each *_perform(). It makes things a lot easier, as the first one that
detects the error get to write the final error reason...

lib/urldata.h

@@ -446,6 +446,9 @@ struct UrlState {
   long sessionage;                  /* number of the most recent session */
 
   char scratch[BUFSIZE*2]; /* huge buffer when doing upload CRLF replacing */
+  bool errorbuf; /* Set to TRUE if the error buffer is already filled in.
+                    This must be set to FALSE every time _easy_perform() is
+                    called. */
 };