This change updates the BSD licence to the three-clause version since
NetBSD has already done so.  This makes dash GPL-compatible.

It also adds Christos Zoulas (NetBSD ash maintainer) to the COPYING file.
I've added "copyright by Herbert Xu" to most files.

Finally all CVS IDs and inclusion of sys/cdefs.h have been removed.
The latter is needed for support of klibc.