Add shebang to make it clear these files are shell scripts (they are
explicitely executed by sh(1)).

discussed with deraadt@

Consistency.

rc.{local,securelevel,shutdown} become examples. If versions of them
are created in /etc, they are executed (they used to be sourced) to
avoid polluting the rc variable space. The powerdown= and securelevel=
features are removed; they are likely only used by 2 people. the
secureleve is now always raised; this is the only sensible default.
ok ajacoutot