tail: fix tailing non seekable files on certain systems

tail: fix tailing non seekable files on certain systems

* src/tail.c (tail_bytes): On systems were blksize_t is unsigned
and the same size or wider than off_t (android for example),
our initialized (off_t) -1 would be promoted to unsigned before
comparison, and thus fail to follow the appropriate path.
* tests/tail-2/tail-c.sh: Add a test case.
* NEWS: Mention the fix.
This issue was introduced in commit v8.23-47-g2662702
Reported at https://github.com/termux/termux-app/issues/233