Add Group ID (gid) functionality to processes

[connorkuehl]

This PR adds the Group ID feature to processes on the system. This addition enables checking group permissions through the has_permissions() function.

In addition to adding the group ID field to the process structure, the following changes are included with this PR:

• spawn_init() has been updated to initialize the GID. Similarly, spawn_process() has been updated to inherit the GID from the parent.

• System calls for setting and getting the process GID have been implemented.

• The chown command has been updated to appropriately submit the current gid instead of the process's uid in its place.

• The chgrp command has been added. It is basically identical to the chown command implementation except it operates on changing the GID.

• The procfs module's status function has been updated to show the process GID.

• The has_permissions() function has been updated to check the GID.

Testing

I didn't see a folder for test programs so I wasn't sure if it was appropriate to even add one in this PR.

I tested this manually with make headless and a small toy program that would set the process GID using the getgid() and setgid() system calls. I ran this as a background process and then checked /proc/pid/status to verify the new GID was there.

Remarks

To the best of my knowledge I believe this PR is complete. I'm still getting familiar with the code base so any feedback on whether this change is incomplete, unnecessary, unwanted, etc, is greatly appreciated.

[klange]

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