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.

Kernel/i386.cpp

@@ -78,6 +78,7 @@ asm( \
     "    pushw %ss\n" \
     "    pushw %ss\n" \
     "    pushw %ss\n" \
+    "    pushw %ss\n" \
     "    popw %ds\n" \
     "    popw %es\n" \
     "    popw %fs\n" \
@@ -85,6 +86,7 @@ asm( \
     "    mov %esp, exception_state_dump\n" \
     "    call exception_" # ec "_handler\n" \
     "    popw %gs\n" \
+    "    popw %gs\n" \
     "    popw %fs\n" \
     "    popw %es\n" \
     "    popw %ds\n" \
@@ -107,6 +109,7 @@ asm( \
     "    pushw %ss\n" \
     "    pushw %ss\n" \
     "    pushw %ss\n" \
+    "    pushw %ss\n" \
     "    popw %ds\n" \
     "    popw %es\n" \
     "    popw %fs\n" \
@@ -114,6 +117,7 @@ asm( \
     "    mov %esp, exception_state_dump\n" \
     "    call exception_" # ec "_handler\n" \
     "    popw %gs\n" \
+    "    popw %gs\n" \
     "    popw %fs\n" \
     "    popw %es\n" \
     "    popw %ds\n" \