tail: only retry file open if --retry specifed

tail: only retry file open if --retry specifed

* src/tail.c (tail_file): On failure to open a file,
set ignore=true when --retry is not specified.
* tests/tail-2/assert-2.sh: Adjust to the new behavior.
* tests/tail-2/retry.sh: Add a test case.  Also change
from `tail ... && fail=1` to the more robust `returns_ 1 ...`
construct which detects segfaults etc.
* NEWS: Document the fix.