kernel: Set kernel threads' initial active directories to avoid taking the first directory they use

This ends up being init's directory, which is fine when init's directory doesn't change...
but a little less fine when the init process calls exec()...
which is what it does in the new configuration I'm testing...

kernel/src/thread/Scheduler.cpp

@@ -86,6 +86,7 @@ namespace Scheduler
         thread->name = name;
 
         thread->is_kernel = true;
+        thread->active_directory = MMU::kernel_page_directory();
 
         thread->auth = Credentials { .uid = 0, .euid = 0, .suid = 0, .gid = 0, .egid = 0, .sgid = 0 };