fix regex, remove unsafe pointer usage, and add graceful HTTP shutdown

- Fix versionMatcher regex: gorilla/mux does not support (?:) syntax,
  and -dirty suffix was required instead of optional
- Replace unsafe.Pointer LUN casts with binary.LittleEndian.Uint64
  in sbc.go, scsi.go, and target.go
- Implement graceful HTTP server shutdown with 5s timeout using
  srv.Shutdown() instead of raw listener close
- Replace golang.org/x/net/context with standard library context
- Respect existing req.Cancel value in canceler to avoid overwriting
- Add early context cancellation check in Do() to fail fast

Based on review of PR #120 by @orzhang, with fixes applied.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

pkg/apiserver/apiserver.go

@@ -19,11 +19,13 @@ package apiserver
 
 import (
 	"crypto/tls"
+	"context"
 	"fmt"
 	"net"
 	"net/http"
 	"strconv"
 	"strings"
+	"time"
 
 	systemdActivation "github.com/coreos/go-systemd/activation"
 	"github.com/docker/go-connections/sockets"
@@ -34,12 +36,11 @@ import (
 	"github.com/gostor/gotgt/pkg/apiserver/router/lu"
 	"github.com/gostor/gotgt/pkg/apiserver/router/target"
 	log "github.com/sirupsen/logrus"
-	"golang.org/x/net/context"
 )
 
 // versionMatcher defines a variable matcher to be parsed by the router
 // when a request is about to be served.
-const versionMatcher = "/v{version:[0-9.]+(?:-dirty)}"
+const versionMatcher = "/v{version:[0-9.]+(-dirty)?}"
 
 // Config provides the configuration for the API server
 type Config struct {
@@ -137,7 +138,9 @@ func (s *HTTPServer) Serve() error {
 
 // Close closes the HTTPServer from listening for the inbound requests.
 func (s *HTTPServer) Close() error {
-	return s.l.Close()
+	ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
+	defer cancel()
+	return s.srv.Shutdown(ctx)
 }
 
 func (s *Server) initTCPSocket(addr string) (l net.Listener, err error) {