tail: fix unlikely races with >=2 --pids

Also, fix commentary to talk about “nonexistent” rather than
“dead” processes, since the code looks for the former not the
latter and the difference matters for zombies.
* src/tail.c (some_writers_exist): Rename from writers_are_dead,
negate the sense, don’t have a special and counterintuitive case
for !nbpids, remove PIDs found not to exist, and avoid some
though not all unlikely races when kernels reuse PIDs.
(tail_forever): Optimize via blocking I/O even if --pid was used,
so long as all the writers no longer exist.
(tail_forever, tail_forever_inotify): Simplify the writers_dead
logic; there is no need to have a local var to track this, since
we can use pids and nbpids now.
(parse_options): Also free and clear pids if !forever.

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