luci-base: add odhcpd feature detection

luci-base: add odhcpd feature detection

For dnsmasq, feature detection is currently supported like this:

	L.hasSystemFeature('dnsmasq', 'dhcpv6')

while for odhcpd, only a basic check is supported:

	L.hasSystemFeature('odhcpd')

With this patch, a similar feature check is also possible for odhcpd,
e.g.:

	L.hasSystemFeature('odhcpd', 'dhcpv6')

Signed-off-by: David Härdeman <[email protected]>