Fix default value of POWERDOWNFLAG

POWERDOWNFLAG path changed from the hard-coded value /etc/killpower to
the build-time generated @CONFPATH@/killpower. This resulted in an
unexpected value '/etc/nut/killpower', at least on Debian.
(reported by Laurent Bigonville)

Closes networkupstools/nut#74
Replace outdated references to shutdown.txt

shutdown.txt was merged into config-notes.txt during the AsciiDoc
conversion of the whole documentation and website. This content is now
available in the docs/config-notes.txt file, section [[UPS_shutdown]]
"Configuring automatic shutdowns for low battery events"