Introduce posix_spawn

[Barath Aron]

Hello again,

I intended to introduce the use of the posix_spawn to the findutils. The 
Threos OS ( https://threos.io ), as you might know, does not support 
fork, but it has posix_spawn. My plan is the following: I write the code 
that implement the process spawning with the posix_spawn, and then the 
only thing left to do is the integration.
Before I start to work on this, I have questions:
(1) Would you push the changes (after review, configuration, etc) to the 
mainline? I don't want to keep the changes as patch files forever, 
because the mainline may diverge.
(2) Do you know any issue from the posix_spawn's perspective? For 
example, some nasty hackery here and there.

As a reference, the GNU make can already uses posix_spawn over fork, 
after I supplied the basic implementation (gathered all required 
information, and I put all together).

Aron