curl: add variables to --write-out

In particular, these ones can help a user to create its own error
message when one or transfers fail.

writeout: add 'onerror', 'url', 'urlnum', 'exitcode', 'errormsg'

onerror - lets a user only show the rest on non-zero exit codes

url - the input URL used for this transfer

urlnum - the numerical URL counter (0 indexed) for this transfer

exitcode - the numerical exit code for the transfer

errormsg - obvious

Reported-by: Earnestly on github
Fixes #6199
Closes #6207

src/tool_sdecls.h

@@ -105,6 +105,7 @@ struct getout {
   char          *outfile;   /* where to store the output */
   char          *infile;    /* file to upload, if GETOUT_UPLOAD is set */
   int            flags;     /* options - composed of GETOUT_* bits */
+  int            num;       /* which URL number in an invocation */
 };
 
 #define GETOUT_OUTFILE    (1<<0)  /* set when outfile is deemed done */