/etc/shells micropolicy

The expected audience of this is debian developers packaging programs
meant to be used as login shells.

/etc/shells is no longer a config file, but is maintained by the
add-shell and remove-shell programs.  So, if a package contains
something that the maintainer thinks ought to be a valid login shell,
it's postinst should, (on initial install only, to allow a sysadmin to
take it out again), run:

/usr/sbin/add-shell /path/to/shell

In the postrm, probably on remove, the package should call

/usr/sbin/remove-shell /path/to/shell

As the various shells start to use it, the default shells list will
shrink.