POSIX::Spawn.system wrong return values and stale $? for missing commands

The MRI `Kernel.system` method returns `nil` if the command could not be found, but `POSIX::Spawn.system` returns `false`.

Simple test:

```ruby
puts Kernel.system("foo").inspect
puts POSIX::Spawn.system("foo").inspect
```

Output:

```
nil
false
```

Looking at the source this should be a one-line fix.