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.

bootstrap.conf

@@ -193,6 +193,7 @@ gnulib_modules="
   physmem
   pipe-posix
   pipe2
+  poll
   posix-shell
   posixtm
   posixver
@@ -227,7 +228,6 @@ gnulib_modules="
   save-cwd
   savedir
   savewd
-  select
   selinux-at
   setenv
   settime