tail: use poll, not select

tail: use poll, not select

This fixes an unlikely stack out-of-bounds write reported by
Stepan Broz via Kamil Dudka (Bug#49209).
* bootstrap.conf (gnulib_modules): Replace select with poll.
* src/tail.c: Do not include <sys/select.h>.
[!_AIX]: Include poll.h.
(check_output_alive) [!_AIX]: Use poll instead of select.
(tail_forever_inotify): Likewise.  Simplify logic, as there is no
need for a ‘while (len <= evbuf_off)’ loop.