unix: update z/OS implementation of fcntl and mmap

- Add a wrapper function around fcntl to handle different operation
  types and new fcntl implementation that accepts uintptr as an arg.
- Add support for calling mmap/munmap with address pointers.
- Add accompanying tests for new functions.

Change-Id: If5e77aa4cf2cccfd431de4f3bd0c5014a761e167
GitHub-Last-Rev: 07e32a4ab797c7baffdb50f8407cf679cabf1dae
GitHub-Pull-Request: golang/sys#216
Reviewed-on: https://go-review.googlesource.com/c/sys/+/610296
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Auto-Submit: Dmitri Shuralyov <dmitshur@golang.org>
TryBot-Bypass: Dmitri Shuralyov <dmitshur@golang.org>

unix/mmap_zos_test.go

@@ -72,3 +72,17 @@ func TestMmap(t *testing.T) {
 		t.Fatalf("Munmap: %v", err)
 	}
 }
+
+func TestMmapPtr(t *testing.T) {
+	p, err := unix.MmapPtr(-1, 0, nil, uintptr(2*unix.Getpagesize()),
+		unix.PROT_READ|unix.PROT_WRITE, unix.MAP_ANON|unix.MAP_PRIVATE)
+	if err != nil {
+		t.Fatalf("MmapPtr: %v", err)
+	}
+
+	*(*byte)(p) = 42
+
+	if err := unix.MunmapPtr(p, uintptr(2*unix.Getpagesize())); err != nil {
+		t.Fatalf("MunmapPtr: %v", err)
+	}
+}