Refactor common parts of gluon-mesh-vpn-fastd into a gluon-mesh-vpn-core package

docs/site-example/site.conf

docs/user/site.rst

package/gluon-mesh-vpn-core/Makefile

package/gluon-mesh-vpn-core/check_site.lua

...uon-mesh-vpn-core/files/lib/gluon/mesh-vpn/iptables.rules

...gluon-mesh-vpn-core/luasrc/lib/gluon/upgrade/400-mesh-vpn

package/gluon-mesh-vpn-fastd/Makefile

package/gluon-mesh-vpn-fastd/check_site.lua

...h-vpn-fastd/files/lib/gluon/mesh-vpn-fastd/iptables.rules

package/gluon-mesh-vpn-fastd/files/lib/gluon/mesh-vpn/fastd

...esh-vpn-fastd/luasrc/lib/gluon/upgrade/400-mesh-vpn-fastd

...std/luasrc/lib/gluon/upgrade/420-mesh-vpn-fastd-simple-tc

package/gluon-web-mesh-vpn-fastd/check_site.lua

...fastd/luasrc/lib/gluon/web/model/admin/mesh_vpn_fastd.lua