[Toybox] [PATCH] tail: implement -F (and its companion -s).

[enh]

(Based on someone else's patch.)

Implementing -F with inotify is a lot more work (including more
portability shims for macOS), so this is a simpler polling
implementation.

Also fix my earlier mistake where xnotify_add() wasn't actually an 'x'
function that exits on failure.
---
 lib/portability.c |   4 +-
 toys/posix/tail.c | 117 +++++++++++++++++++++++++++++++++++++---------
 2 files changed, 96 insertions(+), 25 deletions(-)
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