let parent process wait

[alx-sch]

When piping, the parent process (providing the prompt), waits until all child processes / cmd executions are finished:

:~$ cat | cat | ls
private  repos [--> ls printout]
 [press Enter -> ends cat]
 [press Enter -> end cat]   
aschenk@c3a7c2:~/Documents$ 

However, in our minishell, the parent goes ahead and already prints the new prompt before all processes wrapped up:

🌈 minishell$ cat | cat | ls
private  repos
🌈 minishell$ [need to press ENTER / any other key x2 in order to 'release' prompt input]

Solution: Parent process only waits for the the last child process to terminate, make it wait for all before continuing.