Kernel+LibC: Add a dump_backtrace() syscall.

This is very simple but already very useful. Now you're able to call to
dump_backtrace() from anywhere userspace to get a nice symbolicated
backtrace in the debugger output. :^)

Kernel/Syscall.cpp

@@ -291,6 +291,8 @@ static u32 handle(RegisterDump& regs, u32 function, u32 arg1, u32 arg2, u32 arg3
     case Syscall::SC_reboot: {
         return current->process().sys$reboot();
     }
+    case Syscall::SC_dump_backtrace:
+        return current->process().sys$dump_backtrace();
     default:
         kprintf("<%u> int0x82: Unknown function %u requested {%x, %x, %x}\n", current->process().pid(), function, arg1, arg2, arg3);
         return -ENOSYS;