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/scsi/target.go

@@ -17,8 +17,8 @@ limitations under the License.
 package scsi
 
 import (
+	"encoding/binary"
 	"fmt"
-	"unsafe"
 
 	"github.com/google/uuid"
 	"github.com/gostor/gotgt/pkg/api"
@@ -97,7 +97,7 @@ func RemoveITNexus(target *api.SCSITarget, itnexus *api.ITNexus) {
 
 func deviceReserve(cmd *api.SCSICommand) error {
 	var lu *api.SCSILu
-	lun := *(*uint64)(unsafe.Pointer(&cmd.Lun))
+	lun := binary.LittleEndian.Uint64(cmd.Lun[:])
 
 	for tgtLUN, lunDev := range cmd.Target.Devices {
 		if tgtLUN == lun {