It makes the most sense to have all our utility functions reside in one place.
There was nothing in corp that could not reasonably live in OSS.
I also updated `StartProcessAsChild` to no longer depend on `futureexec`,
thus reducing the amount of code that needed migration. I tested this change
with `tswin` and it is working correctly.
I have a follow-up PR to remove the corresponding code from corp.
The migrated code was mostly written by @alexbrainman.
Sourced from corp revision 03e90cfcc4dd7b8bc9b25eb13a26ec3a24ae0ef9
Signed-off-by: Aaron Klotz <aaron@tailscale.com>
82cd98609f