We migrate to dnsmasq-full, while disabling most of its features.
Notably, dhcp and dnssec are compiled in, while other features of the
full variant are deselected.

This removes PKG_VERSION and PKG_RELEASE from most Makefiles, as the
value was never useful for Gluon packages; instead, PKG_VERSION is set
to 1 in gluon.mk.

It also removes two other weird definitions:

- gluon-iptables-clamp-mss-to-pmtu replicating the old PKG_VERSION logic
  from gluon-core, but without the fixed PKG_BUILD_DIR to prevent
  unnessary rebuilds
- gluon-hoodselector set GLUON_VERSION=3

Specify conffiles for our packages, so they aren't overwritten during
opkg updates. While this only matters during development, it is
unintended to have different behaviour for opkg update and full firmware
updates.

The package rebuild logic of OpenWrt only checks regular files. Avoid
symlinks in the files directory to make rebuilds more robust.

As PROVIDES can be used to replace real packages now, we don't need the
virtual packages as workaround anymore. This also means that the providing
packages don't need to be added to site.mk explicitly anymore when the
default provider is used.

Nothing outside the config mode depends on luci-base anymore.

This will require site.mk to explicitly pull in the gluon-setup-mode and
gluon-config-mode-core packages.

By setting setup_mode.skip to true in site.conf the device will skip
setup mode on first boot.

gluon-config-mode retains the luci frontend, all setup scripts and config is
moved to the new package.