(strtoumax): Declare if not declared.

(strtoumax): Declare if not declared.
(skip_to_page, first_page_number, last_page_number, page_number,
first_last_page, print_header):
Use uintmax_t for page numbers.
(first_last_page): Remove unnecessary forward declaration.
Do not modify arg (it is now a const pointer).
Return a true if successful, false (without print a diagnostic)
otherwise.
(main): If +XXX does not specify a valid page range, treat it
as a file name.  This follows the response to Open Group XCU ERN 41
<http://www.opengroup.org/sophocles/show_mail.tpl?source=L&listname=austin-group-l&id=7717>,
which says the behavior is allowed.
(skip_to_page): When starting page number exceeds page count,
print both numbers in the diagnostic.
(print_header): Detect page number overflow.