[Toybox] toybox: xargs: add actual support for -P max-proc flag

[Mark Salyzyn]

On 7/31/20 12:30 PM, Mark Salyzyn wrote:
> -P flag was fake just to provide compatibility.  Add support for -P
> maximum process count to actually create parallelism.  Does not
> support SIGUSR1 or SIGUSR2 increment/decrement signals as publicly
> documented for other variants.  If max-proc is 0, run as many
> processes as possible simultaneously.
>
The Bug: referenced were scrubbed, and regardless hides some history. 
When using toybox xargs for a kernel build, it added 5 minutes to the 
total build time when it ignored the -P8 flag and exec'd only one at a time.

Sincerely -- Mark Salyzyn