[Issue 14770] std.process should use lightweight forks where available

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=14770

--- Comment #6 from anonymous4 <dfj1esp02 at sneakemail.com> ---
Maybe have a branch for linux and call clone there, call posix_spawn or fork
for other systems. vfork looks scary and posix deprecated it.

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