…om all leases

* Rationale

Many real-world containers attach to multiple networks (e.g., management + data).
Hardcoding eth0 leaves other links unconfigured and DNS incomplete when different
uplinks provide additional resolvers/search domains.

* Problem

The DHCP helper in main_forknet.go was hardcoded to only operate on eth0.
Containers with multiple network interfaces left additional interfaces unconfigured.
resolv.conf was generated from a single lease, effectively "last-writer-wins".

* Key changes

  1. Enumerate container interfaces:
    - Iterate all non-loopback interfaces and bring each up before starting DHCP.

  2. Start DHCP per interface:
    - For each interface, spawn both DHCPv4 and DHCPv6 clients (dhcpRunV4 and dhcpRunV6)
      in separate goroutines.
    - Continue configuring other interfaces even if bringing one up fails.
    - Wait for all spawned DHCP goroutines to report.

  3. Track leases per interface:
    - Replace single-lease fields with per-interface maps on cmdForknet:
      dhcpv4Leases map[string]*nclient4.Lease and dhcpv6Leases map[string]*dhcpv6.Message
    - Update dhcpRunV4/dhcpRunV6 to store leases keyed by interface name.

  4. Aggregate DNS across all leases:
    - Rewrite dhcpApplyDNS to merge nameservers and search domains from all active
      DHCPv4 and DHCPv6 leases.
    - Deduplicate nameservers and search entries; prefer "search" line when available,
      otherwise write a single "domain".

  5. Concurrency and safety:
    - Keep applyDNSMu to protect access to the lease maps and resolv.conf updates.
    - Logging includes the interface name for clearer diagnostics.

* Behavioral notes

  - Single-interface containers keep working as before (now via enumeration).

  - Multi-interface containers will attempt to acquire addresses and routes on all
    non-loopback interfaces.

  - resolv.conf now contains the union of nameservers and search domains coming from all
    received leases.

  - Default route behavior remains per-interface; no explicit route metrics are
    introduced here.

Signed-off-by: Andrey Mozharovsky <andrii.mozharovskyi@virtuozzo.com>