tail: allow multiple PIDs

tail: allow multiple PIDs

tail can watch multiple files, but currently only a single writer. It
can be useful to watch files from multiple writers, or even processes
not directly related to the files (e.g. watch log files written by a
server process, for the duration of a test driven by a separate
client).

* src/tail.c (writers_are_dead): New function.
(tail_forever): Use it to wait for writers.
(tail_forever_inotify): As above.
(parse_options): Manage --pid options in an array.
* doc/coreutils.texi: Update documentation.
* tests/tail/pid.sh: Add a variant with two PIDs.
* News: Mention the new feature.