Lots of hacking:

- Turn Keyboard into a CharacterDevice (85,1) at /dev/keyboard.
- Implement MM::unmapRegionsForTask() and MM::unmapRegion()
- Save SS correctly on interrupt.
- Add a simple Spawn syscall for launching another process.
- Move a bunch of IO syscall debug output behind DEBUG_IO.
- Have ASSERT do a "cli" immediately when failing.
  This makes the output look proper every time.
- Implement a bunch of syscalls in LibC.
- Add a simple shell ("sh"). All it can do now is read a line
  of text from /dev/keyboard and then try launching the specified
  executable by calling spawn().

There are definitely bugs in here, but we're moving on forward.

LibC/stdio.cpp

@@ -141,7 +141,8 @@ extern "C" {
 
 int putchar(int ch)
 {
-    return ch;
+    Syscall::invoke(Syscall::PutCharacter, ch);
+    return (byte)ch;
 }
 
 int printf(const char* fmt, ...)