Add posix_spawn support

[Leont]

This fixes #15355

This is not yet including the optimization for single-arg system() that doesn't contain metacharacter. I should probably add that before undrafting this.

It also doesn't include Configure probes for detecting if the platform supports posix_spawn, partly because I haven't looked into how to write that, but also because that may be a contentious change and we're quite close to that deadline: it may be a better change for 5.37.1

[jkeenan]

This fixes #15355

This is not yet including the optimization for single-arg system() that doesn't contain metacharacter. I should probably add that before undrafting this.

It also doesn't include Configure probes for detecting if the platform supports posix_spawn, partly because I haven't looked into how to write that, but also because that may be a contentious change and we're quite close to that deadline: it may be a better change for 5.37.1

@Leont, can we get an update on the status of this p.r.? It has acquired merge conflicts. Thanks.