Remove some arbitrary restrictions on size fields,

Remove some arbitrary restrictions on size fields,
so that commands like "sort -k 18446744073709551616" no longer fail merely
because 18446744073709551616 doesn't fit in uintmax_t.  The trick is that
these fields can all be treated as effectively infinity;  their exact
values don't matter, since no internal buffer can be that long.
* src/join.c (string_to_join_field): Verify that SIZE_MAX <= ULONG_MAX
if the code assumes this.  Silently truncate too-large values to SIZE_MAX,
as the remaining code will do the right thing in this case.
* src/sort.c (parse_field_count): Likewise.
* src/uniq.c (size_opt, main): Likewise.
* tests/join/Test.pm (bigfield): New test.
* tests/sort/Test.pm (bigfield): New test.
* tests/uniq/Test.pm (121): New test.
Signed-off-by: Jim Meyering <jim@meyering.net>