[meshnet] Manage system-level kernel and link tunables for packet performance

[kraney]

Note: this PR stacks on top of #733 . Please merge that one first (which should make the shared commits disappear from this PR.) The incremental change of this PR starts with sha:b3c915457042d606dd0dd9bb659ed1ea619af81e.

    Implement kernel sysctl tuning in meshnetd for high-density, high-throughput network meshes.

    Reason for Change:
    - Default Linux link queue lengths, netdev backlogs, multicast membership limits, and
      strict rp_filter settings cause packet drops during netdev creation bursts and multi-link tests.
    - IPv6 DAD (Duplicate Address Detection) and Router Solicitations introduce 1-2s readiness
      delays per netdev when launching virtual topologies.

    Key Changes:
    - System & Kernel Tuning (`utils/wireutil/sys_tune.go`):
      - Sets `RLIMIT_NOFILE` to 1,048,576 open file descriptors.
      - Sets `netdev_max_backlog` to 10,000 for device queue bursts.
      - Sets OS send/recv max & default buffers (`rmem_max`, `wmem_max`, `rmem_default`, `wmem_default`) to 16 MB.
      - Sets ARP/neighbor GC thresholds (`gc_thresh1`=1024, `gc_thresh2`=4096, `gc_thresh3`=8192).
      - Sets `igmp_max_memberships` to 10,000 and `mld_max_msf` to 4,096 to prevent multicast group netdev drops.
      - Sets `rp_filter` to 2 (Loose mode) to allow asymmetric routing across multi-link meshes.
      - Disables IPv6 DAD (`accept_dad`=0) and RS floods (`router_solicitations`=0) for instant link readiness.
      - Sets `ipv6/route/max_size` to 1,048,576 for large routing tables.
      - Configured link `txqueuelen` (default 10,000) across TAP, vEth, and vxLAN interface creation.
      - All tunables are overridable via environment variables without recompilation.

[kraney]

FWIW, I created a throughput test comparing before & after for grpcwire. ("Before" being before #731 , "after" being after #734 , including the full set of improvements.)

Before:

After: