* src/file-priv.h (struct _GnumFileSaver):
Added "overwrite_files" field.

* src/file.h (gnum_file_saver_set_overwrite_files): New function.

* src/file.c
(gnum_file_saver_init): Initialize overwrite_files (set to TRUE).
(gnum_file_saver_save): Report an error and abort saving if file with
given name already exists and overwrite_files == FALSE.

* src/plugin-service.h (struct _PluginServiceFileSaver):
Added "overwrite_files" field.

* src/plugin-service.c (plugin_service_file_saver_read,
gnum_plugin_file_saver_new): Handle overwrite_files flag.

* src/xml-io.c (gnumeric_xml_read_workbook):
Use fstat to get file size. Corrected error message.