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/Syscall.cpp

@@ -22,6 +22,7 @@ asm(
     "    pushw %ss\n"
     "    pushw %ss\n"
     "    pushw %ss\n"
+    "    pushw %ss\n"
     "    popw %ds\n"
     "    popw %es\n"
     "    popw %fs\n"
@@ -29,6 +30,7 @@ asm(
     "    mov %esp, syscallRegDump\n"
     "    call syscall_entry\n"
     "    popw %gs\n"
+    "    popw %gs\n"
     "    popw %fs\n"
     "    popw %es\n"
     "    popw %ds\n"
@@ -58,15 +60,16 @@ DWORD handle(DWORD function, DWORD arg1, DWORD arg2, DWORD arg3)
         //kprintf("syscall: sleep(%d)\n", arg1);
         current->sys$sleep(arg1);
         break;
+    case Syscall::Spawn:
+        return current->sys$spawn((const char*)arg1);
     case Syscall::PosixOpen:
-        Task::checkSanity("syscall");
-        kprintf("syscall: open('%s', %u)\n", arg1, arg2);
+        //kprintf("syscall: open('%s', %u)\n", arg1, arg2);
         return current->sys$open((const char*)arg1, (size_t)arg2);
     case Syscall::PosixClose:
-        kprintf("syscall: close(%d)\n", arg1);
+        //kprintf("syscall: close(%d)\n", arg1);
         return current->sys$close((int)arg1);
     case Syscall::PosixRead:
-        kprintf("syscall: read(%d, %p, %u)\n", arg1, arg2, arg3);
+        //kprintf("syscall: read(%d, %p, %u)\n", arg1, arg2, arg3);
         return current->sys$read((int)arg1, (void*)arg2, (size_t)arg3);
     case Syscall::PosixSeek:
         // FIXME: This has the wrong signature, should be like lseek()