Remove arbitrary restrictions on sizes, fixing a bug reported

Remove arbitrary restrictions on sizes, fixing a bug reported
by Geoff Whale.

(skip_fields, skip_chars, check_chars): Now size_t, not int.
(size_opt): New function.  Do not arbitrarily reject size zero.
Change the wording of the error message slightly, for convenience.
(find_field): Use size_t, not int, to compute sizes.
(different, main): check_chars==0 is no longer a special case, as
it defaults to SIZE_MAX.
(main): Check for overflow with args like -234234234234234.
Use 'size_opt' to convert optional arguments to sizes.