tac: handle short reads on input

tac: handle short reads on input

This can be reproduced by getting the read() above 2G,
which induces a short read, thus triggering the erroneous failure.

  $ truncate -s 5G 5G

  $ cat 5G | TMPDIR=$PWD tac | wc -c
  tac: /tmp/tacFt7txA: read error: Illegal seek
  0

With the fix in place we now get:

  $ cat 5G | TMPDIR=$PWD src/tac | wc -c
  5368709120

* src/tac.c (tac_seekable): Use full_read() to handle short reads.
* NEWS: Mention the bug fix.
Reported at https://bugs.debian.org/1042546