unix: add Exec call

The syscall execve has no wrapper in this library, which it seems like
it should - Ian seemed to concur in CL 72550.

Add a docstring and an example, because I always get confused about
how to invoke the syscall.

Change-Id: I6100bbbf4ace9e3e341bf186a04cc03301da9aea
Reviewed-on: https://go-review.googlesource.com/101282
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

unix/example_test.go

@@ -0,0 +1,16 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package unix
+
+import (
+	"log"
+	"os"
+	"syscall"
+)
+
+func ExampleExec() {
+	err := syscall.Exec("/bin/ls", []string{"ls", "-al"}, os.Environ())
+	log.Fatal(err)
+}