tail: fix --follow to not use inotify on remote files

tail: fix --follow to not use inotify on remote files

* src/tail.c (struct File_spec): Add a flag to record if file is remote.
(recheck): If we're using inotify then check if the file has gone remote
and if so, drop it with a warning.
(any_remote_files): A new function to check for any open remote files.
(tailable_stdin): A new function to refactor the check for whether
a tailable file was specified through stdin.
(fremote): A new function to check if a file descriptor
refers to a remote file.
(tail_forever_inotify): Add some comments.
(tail_file): Record if a file is remote when initially opened.
(main): Disable inotify if any remote files specified.
Also document the caveat about remounted files not
being noticed by inotify.
* NEWS: Mention the fix.