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

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.