Integrate gracehttp for zero-downtime HTTP restarts

[snadrus]

Summary

Integrates github.com/facebookgo/grace/gracehttp for zero-downtime HTTP restarts across all Curio HTTP listeners (RPC, Web GUI, and market/public HTTPS).

• All HTTP servers are now served through a single gracehttpsvc.Serve() call in rpc.ListenAndServe, enabling graceful handoff of listener file descriptors on restart.
• Restart triggers (SIGUSR2):
  • curio run when another instance is already running (pid file check) — triggers restart and exits 0
  • curio cli restart — remote restart via new RPC Restart API (or --local via pid file)
  • Cluster restart (harmony_machines.restart_request) — sends SIGUSR2 after tasks drain; falls back to exit 100 for systemd if signal fails
  • systemctl reload curio — new ExecReload=/bin/kill -USR2 $MAINPID in the systemd unit
• Shutdown (SIGTERM): curio cli stop and RPC Shutdown now trigger graceful HTTP drain via SIGTERM.
• Pre-restart hook drains the task engine before forking the successor process.
• PID file written early to $CURIO_REPO_PATH/curio.pid for duplicate-start detection and local restarts.

Duplicate start behavior

Running curio run while Curio is already running for the same repo no longer fails with port conflicts. Instead:

1. Read $CURIO_REPO_PATH/curio.pid
2. If the process is alive, send SIGUSR2 and exit successfully
3. Otherwise claim the pid file and start normally

Skipped when:

• LISTEN_FDS is set (gracehttp handoff child)
• --halt-after-init is passed
• CURIO_SKIP_ALREADY_RUNNING=1 is set

Integration tests are unaffected because they use StartCurioHarness, which calls tasks.StartTasks + rpc.ListenAndServe directly and never goes through runCmd.

Usage

# Starting while already running triggers restart
curio run

# Remote restart (via RPC, same as stop)
curio cli restart

# Local restart via pid file
curio cli restart --local

# Systemd zero-downtime reload
systemctl reload curio

RestartForceExitStatus=100 is retained as a fallback when graceful restart cannot signal the process.