Fix race condition in sandbox

[Keno]

The current sandbox code has a race condition. If the child exits before the sigwait signal mask is setup, we will hang forever waiting for a signal. This has been observed in practice in Yggdrasil. I don't know that the sigwait here is supposed to do - plain waitpid should be fine, so try removing it to see if everything works.

If the sigwait does turn out to be necessary, we should be able to fix this by moving the signal mask setup to above the fork instead.

[staticfloat]

I'm pretty sure this code is vestigial from the olden days when this C code was used to setup a fake init for a QEMU-emulated environment for BinaryBuilder.jl. Because it was acting as init, it needed to react to SIGCHILD to properly take care of orphaned zombie processes. So I think removing it should be fine.

[codecov]

Codecov Report

All modified lines are covered by tests ✅

Comparison is base (949bf17) 78.68% compared to head (aa73670) 78.68%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #113   +/-   ##
=======================================
  Coverage   78.68%   78.68%           
=======================================
  Files           6        6           
  Lines         624      624           
=======================================
  Hits          491      491           
  Misses        133      133           

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