Allow splicing Eshell globs in-place

Allow splicing Eshell globs in-place

This means that Eshell globs can now expand the same way as if the
user had typed each matching file individually.

* lisp/eshell/em-glob.el (eshell-glob-splice-results): New option.
(eshell-no-command-globbing, eshell-add-glob-modifier): Handle spliced
globs.
(eshell-extended-glob): Always return a list when splicing.

* lisp/eshell/em-pred.el (eshell-parse-arg-modifier): Ensure
'eshell-splice-args' is always at the end of the list of modifiers if
present.

* test/lisp/eshell/em-glob-tests.el
(em-glob-test/expand/splice-results)
(em-glob-test/expand/no-splice-results)
(em-glob-test/expand/explicitly-splice-results)
(em-glob-test/expand/explicitly-listify-results): New tests.
(em-glob-test/no-matches): Check result when
'eshell-glob-splice-results' is nil/non-nil.

* doc/misc/eshell.texi (Arguments): Expand explanation about argument
flattening.
(Globbing): Document splicing behavior of globs.

* etc/NEWS: Announce this change.