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/scsi.go

@@ -21,7 +21,6 @@ import (
 	"encoding/binary"
 	"fmt"
 	"sync"
-	"unsafe"
 
 	"github.com/gostor/gotgt/pkg/api"
 	log "github.com/sirupsen/logrus"
@@ -96,7 +95,7 @@ func (s *SCSITargetService) AddCommandQueue(tid int, scmd *api.SCSICommand) erro
 		}
 	}
 	scmd.ITNexus = itn
-	lun := *(*uint64)(unsafe.Pointer(&scmd.Lun))
+	lun := binary.LittleEndian.Uint64(scmd.Lun[:])
 	scmd.Device = target.Devices[lun]
 
 	log.Debugf("scsi opcode: 0x%x, LUN: %d", int(scmd.SCB[0]), binary.LittleEndian.Uint64(scmd.Lun[:]))