Pre-pause snapshot signal to prevent Go runtime crash on restore

envd crashes with "fatal error: bad summary data" after Firecracker
snapshot/restore because the page allocator radix tree is inconsistent
when vCPUs are frozen mid-allocation. The port scanner goroutine
allocates heavily every second, making it the primary trigger.

Add POST /snapshot/prepare to envd — the host agent calls it before
vm.Pause to quiesce continuous goroutines and force GC. On restore,
PostInit restarts the port subsystem via the existing /init endpoint.

- New PortSubsystem abstraction with Start/Stop/Restart lifecycle
- Context-based goroutine cancellation (replaces irreversible channel close)
- Context-aware Signal to prevent scanner/forwarder deadlock
- Fix forwarder goroutine leak (was spinning forever on closed channel)
- Kill socat children on stop to prevent orphans across snapshots
- Fix double cmd.Wait panic (exec.Command instead of CommandContext)

envd/spec/envd.yaml

@@ -1,4 +1,5 @@
 # SPDX-License-Identifier: Apache-2.0
+# Modifications by M/S Omukk
 
 openapi: 3.0.0
 info:
@@ -70,6 +71,13 @@ paths:
         "204":
           description: Env vars set, the time and metadata is synced with the host
 
+  /snapshot/prepare:
+    post:
+      summary: Quiesce continuous goroutines before Firecracker snapshot
+      responses:
+        "204":
+          description: Goroutines quiesced, safe to snapshot
+
   /envs:
     get:
       summary: Get the environment variables