Be more efficient when buffering output in Eshell

This makes the built-in 'eshell/cat' 5-10x faster on large files in my
(somewhat limited) tests.  In addition, this change periodically
redisplays when using the Eshell buffered output so that users can see
some progress.

* lisp/eshell/esh-io.el (eshell-print-queue-size, eshell-print-queue,
eshell-print-queue-count): Make obsolete in favor of...
(eshell-buffered-print-size, eshell--buffered-print-queue)
(eshell--buffered-print-current-size): ... these.
(eshell-buffered-print-redisplay-throttle): New user option.
(eshell--buffered-print-next-redisplay): New variable.
(eshell-init-print-buffer): Make obsolete.
(eshell-flush): Add new REDISPLAY-NOW argument in favor of CLEAR (which
only 'eshell-init-print-buffer' should have used).
(eshell-buffered-print): Compare queued output length to
'eshell--buffered-print-current-size'.
(eshell-with-buffered-print): New macro.

* lisp/eshell/esh-var.el (eshell/env):
* lisp/eshell/em-dirs.el (eshell/cd):
* lisp/eshell/em-hist.el (eshell/history):
* lisp/eshell/em-unix.el (eshell/cat):
* lisp/eshell/em-ls.el (eshell/ls): Use 'eshell-with-buffered-print'.
(flush-func): Remove.
(eshell-ls--insert-directory, eshell-do-ls): Remove 'flush-func'.

* test/lisp/eshell/em-unix-tests.el (em-unix-test/compile/interactive)
(em-unix-test/compile/pipeline, em-unix-test/compile/subcommand): Fix
indentation.
(em-unix-test/cat/file-output): New test.

* etc/NEWS: Announce these improvements.

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