[meshnet] Replace veth + libpcap with TAP devices for gRPC wires

[kraney]

Replace the veth pair and libpcap packet capture implementation in meshnetd
with persistent Linux TAP devices created directly inside container network
namespaces.

Reason for Change:
- Eliminates packet loss under high traffic loads caused by libpcap's kernel-
  to-userspace drop policy.
- Provides native Linux socket buffer flow control and backpressure (`txqueuelen`)
  up to the container application when gRPC processing falls behind.
- Guarantees protocol transparent packet handling (including LACP and LLDP frames).
- Ensures process crash resilience: setting `TUNSETPERSIST=1` keeps TAP interfaces
  alive in the container netns across daemon restarts without link flaps.
- Synchronizes CNI plugin pod readiness with complete end-to-end gRPC wire setup,
  preventing test/ping race conditions upon pod startup.
- Eliminates CGO and `libpcap-dev` build dependencies, producing a pure Go static
  binary (`CGO_ENABLED=0`).

Key Changes:
- Added `CreateOrAttachTAP` in `wireutil` using `TUNSETIFF` & `TUNSETPERSIST`.
- Replaced `pcap.Handle` with `*os.File` in `gwire_map.go`, `grpcwire.go`, and `handler.go`.
- Updated `ReconcilePodLinks` in `controller.go` to use TAP interfaces directly without
  host-side veth creation.
- Updated `GRPCWireExists` and CNI `cmdAdd` readiness check to block until gRPC wire
  handshakes are fully established.
- Removed `libpcap-dev` and updated Dockerfile to build `meshnetd` with `CGO_ENABLED=0`.