config/shelld is a symbolic link to /etc/shells
which should contain all valid shells for new
users.
Unforutnatly some debian installations don't have
/bin/false and /bin/true in /etc/shells.
It's also possible valid shells differs completly
because /etc/shells on another host should be used.

If you want to use your own list of shells copy /etc/shells
to config/shells and change it to your benefits.

If you have questions feel free to mail me: TiloLutz@gmx.de