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

[Rob Landley]

On 7/31/20 4:19 PM, Mark Salyzyn via Toybox wrote:
> 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

I did a slightly smaller implementation (I'd looked at implementing -P before,
but waited for somebody to complain about its absence), which supports SIGUSR1
and SIGUSR2.

Does what I checked in work for you?

Thanks,

Rob