setting variable on Bourne-shell startup

[Guy Harris]

>The valid values for shell names used with "chsh" are:
>	/bin/sh
>	/bin/csh
>	/bin/ksh

On many systems (including 4.3BSD and later), the valid values for shell
names used with "chsh" are whatever the file "/etc/shells" says, or
"/bin/sh" and "/bin/csh" if that file doesn't exist.  I hope nobody
hardcoded those three into their system, except as a default if
"/etc/shells" doesn't exist....