VRF: T3655: proper connection tracking for VRFs

Currently, all VRFs share the same connection tracking table, which can
lead to problems:

• traffic leaks to a wrong VRF
• improper NAT rules handling when multiple VRFs contain the same IP

networks

• stateful firewall rules issues

The commit implements connection tracking zones support. Each VRF
utilizes its own zone, so connections will never mix up.

It also adds some restrictions to VRF names and assigned table numbers,
because of nftables and conntrack requirements:

• VRF name should always start from a letter (interfaces that start from

numbers are not supported in nftables rules)

• table number must be in the 100-65535 range because conntrack supports

only 65535 zones