Fix a race condition when running Eshell startup scripts

Fix a race condition when running Eshell startup scripts

Previously, these scripts could run before all the Eshell modules had
fully-initialized.

* lisp/eshell/esh-mode.el (eshell-after-initialize-hook): New hook...
(eshell-mode): ... run it.

* lisp/eshell/em-script.el (eshell-run-startup-scripts): New function...
(eshell-script-initialize): ... add it to
'eshell-after-initialize-hook'.

* etc/NEWS: Announce 'eshell-after-initialize-hook'.

(cherry picked from commit 605f26cf70ab3d7c5ea635c19cd2a280812a4ddc)