libstd/sys/unix/process.rs: reap a zombie who didn't get through to exec(2)

After the library successfully called `fork(2)`, the child does several
setup works such as setting UID, GID and current directory before it
calls `exec(2)`.  When those setup works failed, the child exits but the
parent didn't call `waitpid(2)` and left it as a zombie.

This patch also add several sanity checks.  They shouldn't make any
noticeable impact to runtime performance.

The new test case in `libstd/io/process.rs` calls the ps command to check
if the new code can really reap a zombie.
The output of `ps -A -o pid,sid,command` should look like this:

```
  PID   SID COMMAND
    1     1 /sbin/init
    2     0 [kthreadd]
    3     0 [ksoftirqd/0]
...
12562  9237 ./spawn-failure
12563  9237 [spawn-failure] <defunct>
12564  9237 [spawn-failure] <defunct>
...
12592  9237 [spawn-failure] <defunct>
12593  9237 ps -A -o pid,sid,command
12884 12884 /bin/zsh
12922 12922 /bin/zsh
...
```

where `./spawn-failure` is my test program which intentionally leaves many
zombies.  Filtering the output with the "SID" (session ID) column is
a quick way to tell if a process (zombie) was spawned by my own test
program.  Then the number of "defunct" lines is the number of zombie
children.
1.  The [`forget` test](https://github.com/rust-lang/rust/blob/master/src/libstd/io/process.rs#L1176) intentionally leaves a zombie (at least that's how I understood it)
2. If we run the test driver with multiple threads, we cannot guarantee the equality.