unix: fix IoctlGetPtmget get empty ptsname on NetBSD.

On NetBSD, because the size of the Ptmget structure does not match the value of TIOCPTSNAME, IoctlGetPtmget uses an incorrect structure internally, resulting in empty ptsname.

See https://github.com/NetBSD/src/blob/trunk/sys/sys/ttycom.h

Fixes #66871

Author: iyzyi

Diff for: unix/syscall_netbsd_test.go

@@ -38,7 +38,12 @@ func TestIoctlPtmget(t *testing.T) {
 		t.Fatalf("IoctlGetPtmget: %v\n", err)
 	}
 
-	t.Logf("sfd = %v, ptsname = %v", ptm.Sfd, unix.ByteSliceToString(ptm.Sn[:]))
+	ptsname := unix.ByteSliceToString(ptm.Sn[:])
+	if ptsname == "" {
+		t.Fatalf("IoctlGetPtmget: ptsname is empty string\n")
+	}
+
+	t.Logf("sfd = %v, ptsname = %v", ptm.Sfd, ptsname)
 }
 
 func TestStatvfs(t *testing.T) {