tail: fix output of redundant headers when resuming

tail: fix output of redundant headers when resuming

* src/tail.c (check_fspec): Only enable printing of the file header
if we've actually read some data and this is a new file.  Also
move printing of the file header to...
(dump_remainder): ...here, to allow printing only when data read.
* tests/tail-2/overlay-headers.sh: A new test for suspension
and resumption of tail.
* tests/local.mk: Reference the new test.
* NEWS: Mention the fix.
Fixes http://bugs.gnu.org/23539