tail: with -f, use polling when a file is on an FS of unknown type

Before, we would use inotify in that case, which would work as long
as updates were taking place locally, but not at all when remote.
Move hard-coded list of known remote FS types into a more
maintainable table in stat.c, alongside the list of FS
names and magic numbers.  Generate a new is_local_fs_type function.
* src/Makefile.am (fs-is-local.h): New rule, generated file.
* src/extract-magic: Revamp to parse local/remote keyword after
each magic number in src/stat.c's case statements.
Accept new --local option.
* src/.gitignore: Ignore the generated fs-is-local.h.
* src/tail.c [HAVE_INOTIFY]: Include fs-is-local.h.
(fremote) [HAVE_INOTIFY]: Use the new function in place of
the switch stmt with hard-coded list of FS types.
Emit a warning when processing a file on a file system of unknown type.
* NEWS (Changes in behavior): Mention it.
Suggested by Sven Breuner.

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