* src/print.c: Illustrate margin definitions.
(print_headers): Header, not top margin, should be top of header
area.
(print_footers): Footer, not bottom margin, should be top of
footer area.

* src/xml-io.c (xml_print_info_fix_margins): New function fix up
incorrectly stored margins.
(xml_read_print_margins): Use it.

* src/dialogs/dialog-printer-setup.c (do_fetch_margins,
do_setup_margin): Header, not top margin, should be top of header area
and footer, not bottom margin, should be top of footer area.

* plugins/excel/ms-excel-read.c (ms_excel_init_margins): New
function. Initialize margins to Excel defaults.  (ms_excel_sheet_new):
Call ms_excel_init_margins.

* plugins/excel/ms-excel-write.c (write_sheet_bools): Write margins
also for biff 7 (which is what we actually use).