migrate to go.sys subrepository

As of go version 1.4 the standard library syscall package is "locked
down" and code outside of the standard library is recommended to migrate
to the go.sys subrepository.

Reference: https://golang.org/s/go1.4-syscall

proctl/threads.go

@@ -3,7 +3,7 @@ package proctl
 import (
 	"encoding/binary"
 	"fmt"
-	"syscall"
+	sys "golang.org/x/sys/unix"
 
 	"github.com/derekparker/delve/dwarf/frame"
 )
@@ -13,7 +13,7 @@ import (
 type ThreadContext struct {
 	Id      int
 	Process *DebuggedProcess
-	Status  *syscall.WaitStatus
+	Status  *sys.WaitStatus
 }
 
 type Registers interface {
@@ -91,7 +91,7 @@ func (thread *ThreadContext) Continue() error {
 		}
 	}
 
-	return syscall.PtraceCont(thread.Id, 0)
+	return sys.PtraceCont(thread.Id, 0)
 }
 
 // Single steps this thread a single instruction, ensuring that
@@ -122,7 +122,7 @@ func (thread *ThreadContext) Step() (err error) {
 		}()
 	}
 
-	err = syscall.PtraceSingleStep(thread.Id)
+	err = sys.PtraceSingleStep(thread.Id)
 	if err != nil {
 		return fmt.Errorf("step failed: %s", err.Error())
 	}