Update to revision r40649:
Correct the example on how to prevent NIS users from logging in.  The
previous instructions did not work and this corrected version is based on
a working config from a production system.  Add some descriptive text, too.

Submitted by:	Bjoern Heidotting
Obtained from:	The FreeBSD German Documentation Project
Put in the missing revision r40833:
Remove mention of plip0, the parallel-port network device.

Submitted by:	    Bjoern Heidotting
Obtained from:	    The FreeBSD German Documentation Project
Update to r41817:
Woops, this is in the 'Changes from the First Edition' section, not a
generic introduction.

Submitted by:	Bjoern Heidotting
Obtained from:	The FreeBSD German Documentation Project