dns_*_nsupdate sample scripts are broken, and a bad example

[tlhackque]

As noted in #286, the dns_add/del_update sample scripts leave a lot be be desired.
Further investigation found more issues; here is the complete list:

• They are inefficient - use unnecessary temp files
• They require a custom copy of the script for each instance
• The exit status of nsupdate is clobbered by the rm of the tempfile; they never fail
• There is no provision for pre/post processing (e.g. decrypt a key file or mount a disk containing it)

I've rewritten them; there'll be a PR shortly.