bug#25496: 25.1.91; INSIDE_EMACS env variable is not set in eshell

[Alex Hutcheson]

The INSIDE_EMACS environmental variable is set for comint processes,
including M-x shell and ansi-term. However, it's not set for eshell or
for processes launched by eshell. This makes it difficult for scripts to
detect whether or not they are being run inside eshell.

The INSIDE_EMACS env variable should be set within eshell, or should at
least be set for processes launched by eshell.

In GNU Emacs 25.1.91.1 (x86_64-pc-linux-gnu, X toolkit, Xaw3d scroll bars),  
modified by Debian
Windowing system distributor 'The X.Org Foundation', version 11.0.11501000
System Description:     Ubuntu 14.04 LTS

Configured using:
 'configure --build x86_64-linux-gnu --build x86_64-linux-gnu
 --prefix=/usr --sharedstatedir=/var/lib --libexecdir=/usr/lib
 --localstatedir=/var/lib --infodir=/usr/share/info
 --mandir=/usr/share/man --with-pop=yes
  
--enable-locallisppath=/etc/google-emacs:/etc/emacs:/usr/local/share/emacs/25.1.91+gg1+1/site-lisp:/usr/local/share/emacs/site-lisp:/usr/share/emacs/25.1.91+gg1+1/site-lisp:/usr/share/emacs/site-lisp
 --with-crt-dir=/usr/lib/x86_64-linux-gnu --disable-build-details
 --disable-silent-rules --with-modules GOOGLE_VERSION=25.1.91+gg1+1
 --with-x=yes --with-x-toolkit=lucid --with-toolkit-scroll-bars
 --without-gconf --without-gsettings build_alias=x86_64-linux-gnu
 'CFLAGS=-g -O2 -fstack-protector --param=ssp-buffer-size=4 -Wformat
 -Werror=format-security -Wall' 'LDFLAGS=-Wl,-Bsymbolic-functions
 -Wl,-z,relro -Wl,-fuse-ld=gold,--export-dynamic-symbol=__google_auxv'
 'CPPFLAGS=-D_FORTIFY_SOURCE=2 -DGOOGLE_EMACS_DEFINE_AUXV''

Configured features:
XAW3D XPM JPEG TIFF GIF PNG RSVG IMAGEMAGICK SOUND GPM DBUS NOTIFY
LIBSELINUX GNUTLS LIBXML2 FREETYPE M17N_FLT LIBOTF XFT ZLIB
TOOLKIT_SCROLL_BARS LUCID X11 MODULES

Important settings:
  value of $LANG: en_US.UTF-8
  value of $XMODIFIERS: @im=ibus
  locale-coding-system: utf-8-unix

Major mode: Lisp Interaction

Minor modes in effect:
  tooltip-mode: t
  global-eldoc-mode: t
  electric-indent-mode: t
  mouse-wheel-mode: t
  tool-bar-mode: t
  menu-bar-mode: t
  file-name-shadow-mode: t
  global-font-lock-mode: t
  font-lock-mode: t
  blink-cursor-mode: t
  auto-composition-mode: t
  auto-encryption-mode: t
  auto-compression-mode: t
  line-number-mode: t
  transient-mark-mode: t

Recent messages:
For information about GNU Emacs and the GNU system, type C-h C-a.

Load-path shadows:
None found.

Features:
(shadow sort mail-extr emacsbug message dired format-spec rfc822 mml
mml-sec password-cache epg epg-config gnus-util mm-decode mm-bodies
mm-encode mail-parse rfc2231 mailabbrev gmm-utils mailheader sendmail
rfc2047 rfc2045 ietf-drums mm-util help-fns help-mode easymenu
cl-loaddefs pcase cl-lib mail-prsvr mail-utils time-date mule-util
tooltip eldoc electric uniquify ediff-hook vc-hooks lisp-float-type
mwheel x-win term/common-win x-dnd tool-bar dnd fontset image regexp-opt
fringe tabulated-list newcomment elisp-mode lisp-mode prog-mode register
page menu-bar rfn-eshadow timer select scroll-bar mouse jit-lock
font-lock syntax facemenu font-core frame cl-generic cham georgian
utf-8-lang misc-lang vietnamese tibetan thai tai-viet lao korean
japanese eucjp-ms cp51932 hebrew greek romanian slovak czech european
ethiopic indian cyrillic chinese charscript case-table epa-hook
jka-cmpr-hook help simple abbrev minibuffer cl-preloaded nadvice
loaddefs button faces cus-face macroexp files text-properties overlay
sha1 md5 base64 format env code-pages mule custom widget
hashtable-print-readable backquote dbusbind inotify dynamic-setting
font-render-setting x-toolkit x multi-tty make-network-process emacs)

Memory information:
((conses 16 87104 6318)
 (symbols 48 19929 0)
 (miscs 40 51 110)
 (strings 32 15098 4711)
 (string-bytes 1 425436)
 (vectors 16 11832)
 (vector-slots 8 435700 4672)
 (floats 8 168 31)
 (intervals 56 262 0)
 (buffers 976 19)
 (heap 1024 12629 768))