tail: allow >=2**64 in traditional form

This better matches the treatment of POSIX form,
e.g., ‘tail +Nc’ is now like ‘tail -c +N’ even when N is large.
* src/tail.c: Don’t include xstrtol.h.
(parse_obsolete_option): Treat numbers greater than UINTMAX_MAX as
if they are UINTMAX_MAX.  Parse the number by hand with saturating
arithmetic; nowadays that’s simpler than using xstrtoumax.  There
is no need for a diagnostic now, as the error cannot happen any more.
* tests/tail/tail.pl (obs-plus-c3): New test.

tests/tail/tail.pl

@@ -29,6 +29,7 @@ my @tv = (
 #
 ['obs-plus-c1', '+2c', 'abcd', 'bcd', 0],
 ['obs-plus-c2', '+8c', 'abcd', '', 0],
+['obs-plus-c3', '+999999999999999999999999999999999999999999c', 'abcd', '', 0],
 ['obs-c3', '-1c', 'abcd', 'd', 0],
 ['obs-c4', '-9c', 'abcd', 'abcd', 0],
 ['obs-c5', '-12c', 'x' . ('y' x 12) . 'z', ('y' x 11) . 'z', 0],