base-files: Add /etc/shinit for non-login shell init

base-files: Add /etc/shinit for non-login shell init

Because /etc/profile (and ~/.profile) are read by login shells only,
aliases and functions defined there are not available to non-login
shells, e.g. when using screen or tmux.

If the ENV environment variable exists (exported by /etc/profile or
~/.profile) and references an existing file, then all interactive shells
(login or non-login) will read that file as well.

This sets the ENV environment variable in /etc/profile, pointing to
/etc/shinit.

This also adds /etc/shinit, which:

* Contains alias and function definitions originally in /etc/profile

* Sources /etc/mkshrc if the user is using mksh (also originally in
  /etc/profile), as /etc/mkshrc is meant for all interactive shells

* Sources ~/.mkshrc if the user is using mksh, to compensate for the
  fact that mksh will not read ~/.mkshrc if ENV is set

* Sources ~/.shinit if the user is not using mksh

This also removes the shebang from /etc/profile, as the file is sourced,
not executed.

Signed-off-by: Jeffery To <jeffery.to@gmail.com>