kernel: change `SyExecuteProcess` to use `posix_spawn`

[fingolfin]

@ChrisJefferson does this work under cygwin? If so, does it fix the bug where Process is changing the working dir for the parent process?

The patch is not ready for merging in any case:

• handling of SIGCHLD not integreated
• moving SyExecuteProcess to iostream.c is kind of a hack, but for the sake of quickly experimenting I didn't want to bother with trying to come up with a "clean" solution.

[ChrisJefferson]

This works on cygwin, and stops the directory changing behaviour

[fingolfin]

I decided to just make the move of SyExecuteProcess to iostream.c permanent, there seems little point in trying to abstract this away and it makes code sharing much easier.