Copy INSIDE_EMACS env variable to subprocesses in Eshell (Bug#25496)

Copy INSIDE_EMACS env variable to subprocesses in Eshell (Bug#25496)

* lisp/eshell/em-dirs.el (eshell-dirs-initialize): Add INSIDE_EMACS
variable to buffer-local value of eshell-variable-aliases-alist.
(eshell-inside-emacs): Add new constant used for INSIDE_EMACS.
* lisp/eshell/esh-var.el (eshell-variable-aliases-list): Update doc
string; remove mention of eshell-user-aliases-list and explain that
variables can optionally be copied to subprocesses' environments.
* test/lisp/eshell/eshell-tests.el (eshell-test/inside-emacs-var): Add
test for the INSIDE_EMACS variable.
* etc/NEWS: Announce changes.