Clean up file-size-function

Clean up file-size-function

It is now called `byte-count-to-string-function', and used instead of
calling `file-size-human-readable' directly where appropriate.

* lisp/files.el (file-size-human-readable-iec): New.
(file-size-function): Rename to byte-count-to-string-function.  Better
default value.  Eliminate lambda.  Better default for custom choice.
Put in group `files'.  More descriptive doc string.  Move.
(out-of-memory-warning-percentage, warn-maybe-out-of-memory)
(get-free-disk-space):
* lisp/dired.el (dired-number-of-marked-files):
* lisp/url/url-http.el (url-http-simple-after-change-function)
(url-http-content-length-after-change-function):
Use byte-count-to-string-function.
* test/lisp/files-test.el (files-test-file-size-human-readable):
Test file-size-human-readable-iec.