Fix child process spawning on linux

[FrogTheFrog]

Description

This solves a bug introduced accidentally by #600.

When starting an attached command/process on linux, you are welcomed by this error:

terminate called after throwing an instance of 'boost::process::process_error'
  what():  killpg(2) failed in terminate: Invalid argument
Aborted (core dumped)

It seems that for whatever reason, if your child is going to be in a process group, it has to be in it from the start boostorg/process#164 in order for the group methods to work properly.

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Dependency update (updates to dependencies)
• Documentation update (changes to documentation)
• Repository update (changes to repository files, e.g. .github/...)

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have added or updated the in code docstring/documentation-blocks for new or existing methods/components

Branch Updates

LizardByte requires that branches be up-to-date before merging. This means that after any PR is merged, this branch
must be updated before it can be merged. You must also
Allow edits from maintainers.

• I want maintainers to keep my branch updated

[CLAassistant]

All committers have signed the CLA.

[FrogTheFrog]

Note: I don't really like this myself that there is a need to pass a temporary group, but I don't know how to do it optionally without creating 2 function versions. The boost::child constructor is just meh...

[FrogTheFrog]

Got an advice that temporary group could have had some strange behavior on Windows, so duplicating code instead :/

[FrogTheFrog]

@cgutman Sorry to tag you like this, but could I ask for a review from you? Since I know you're more knowledgeable about boost::process stuff?

[FrogTheFrog]

It seems that ctor does not accept PID with other arguments. Since it was working this way on Windows before and it's only broken on linux, the child can be added to the group manually via method.