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.

tests/tail/tail.pl

@@ -68,15 +68,11 @@ my @tv = (
 ['err-4', '-2cX', '', '', 1,
  "$prog: option used in invalid context -- 2\n"],
 
-# Since the number is larger than 2^64, this should provoke
-# the diagnostic: 'tail: 99999999999999999999: invalid number of bytes'
-# on all systems... probably, for now, maybe.
-['err-5', '-c99999999999999999999', '', '', 1,
- "$prog: invalid number of bytes: '99999999999999999999'\n",
- $normalize_strerror],
 ['err-6', '-c --', '', '', 1,
  "$prog: invalid number of bytes: '-'\n", $normalize_strerror],
 
+['big-c', '-c99999999999999999999', '', '', 0],
+
 # Same as -n 10
 ['minus-1', '-', '', '', 0],
 ['minus-2', '-', "x\n" . ("y\n" x 10) . 'z', ("y\n" x 9) . 'z', 0],