Kernel: Introduce threads, and refactor everything in support of it.

The scheduler now operates on threads, rather than on processes.
Each process has a main thread, and can have any number of additional
threads. The process exits when the main thread exits.

This patch doesn't actually spawn any additional threads, it merely
does all the plumbing needed to make it possible. :^)

Kernel/kprintf.cpp

@@ -8,7 +8,11 @@
 
 static void console_putch(char*&, char ch)
 {
-    Console::the().write(*current, (byte*)&ch, 1);
+    if (!current) {
+        IO::out8(0xe9, ch);
+        return;
+    }
+    Console::the().write(current->process(), (byte*)&ch, 1);
 }
 
 int kprintf(const char* fmt, ...)