tail: support counts > 2**64

tail: support counts > 2**64

* src/tail.c (tail_lines): If skipping all input, use lseek if
possible.
(parse_options): Allow counts to exceed 2**64.
(main): Don’t subtract 1 from UINTMAX_MAX, since it stands
for infinity in this context.
(main): Also don’t read anything when given infinite elisions.
* tests/tail/tail.pl: Adjust to match new behavior.  Rename err-5
test to big-c and expect the invocation to succeed, since ‘tail
-c99999999999999999999’ now succeeds instead of (unnecessarily)
failing.