mime: remove support "-" stdin pseudo-file name in curl_mime_filedata().

This feature is badly supported in Windows: as a replacement, a caller has
to use curl_mime_data_cb() with fread, fseek and possibly fclose
callbacks to process opened files.

The cli tool and documentation are updated accordingly.

The feature is however kept internally for form API compatibility, with
the known caveats it always had.

As a side effect, stdin size is not determined by the cli tool even if
possible and this results in a chunked transfer encoding. Test 173 is
updated accordingly.

docs/examples/smtp-mime.c

@@ -153,6 +153,9 @@ int main(void)
      * clean up in the end.
      */
     curl_easy_cleanup(curl);
+
+    /* Free multipart message. */
+    curl_mime_free(mime);
   }
 
   return (int)res;

docs/libcurl/curl_mime_filedata.3

@@ -35,7 +35,6 @@ contents.
 \fIpart\fP is the part's to assign contents to.
 \fIfilename\fP points to the nul-terminated file's path name. The pointer can
 be NULL to detach previous part contents settings.
-If \fIfilename\fP is "-", part contents data will be read from stdin.
 Filename storage can be safely be reused after this call.
 
 As a side effect, the part's remote file name is set to the base name of the