dnsmasq: fix dependency problems of the dnsmasq-full variant.

This patch tries to

 - Let the DHCPv6 feature depend on CONFIG_IPV6.
 - Conditionally select libnettle, kmod-ipv6, kmod-ipt-ipset only if the
   corresponding features are enabled.
 - Install `trust-anchors.conf` only if DNSSEC is selected.
 - Add PKG_CONFIG_DEPENDS for the configurable options.
 - Add a patch to let the Makefile of dnsmasq be aware of changes in
   COPTS variable.

Big thanks goes to Frank Schäfer <fschaefer.oss@googlemail.com> for
providing necessary information on connections and dependency relations
between these CONFIGs and packages.

Signed-off-by: Yousong Zhou <yszhou4tech@gmail.com>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@43851 3c298f89-4303-0410-b956-a3cf2f4a3e73