Kernel: Add sid and pgid to Credentials

There are places in the kernel that would like to have access
to `pgid` credentials in certain circumstances.

I haven't found any use cases for `sid` yet, but `sid` and `pgid` are
both changed with `sys$setpgid`, so it seemed sensical to add it.

In Linux, `man 7 credentials` also mentions both the session id and
process group id, so this isn't unprecedented.

Kernel/Syscalls/execve.cpp

@@ -540,7 +540,9 @@ ErrorOr<void> Process::do_exec(NonnullLockRefPtr<OpenFileDescription> main_progr
                 new_egid,
                 new_suid,
                 new_sgid,
-                old_credentials->extra_gids()));
+                old_credentials->extra_gids(),
+                old_credentials->sid(),
+                old_credentials->pgid()));
         }
     }