config.c: add SKIP_ARPDETECT to ignored list

vzctl warns about unknown parameters in configuration file.
Since we are now using SKIP_ARPDETECT in vz.conf (see previous
patch), it complains:

	Warning at /etc/vz/vz.conf:63: unknown parameter
	SKIP_ARPDETECT ("yes"), ignored

The reason is we don't read SKIP_ARPDETECT from vzctl, it is used
by vps-net_add script which sources vz.conf.

So, add SKIP_ARPDETECT to well known list of parameters to avoid
the warning.

Signed-off-by: Kir Kolyshkin <kir@openvz.org>