Fixes two bugs with keyboard_string (double-letters and backspace lag)

These are pretty easy to grok:
1) Line 92 was clearly left over by accident; line 96 does the work in the "else" branch. This led to letters being "typed" twice.
2) Line 93 checked keyboard_lastkey, but that hadn't actually been set yet. Thus, vk_backspace wouldn't work, or it *would* work, but 1 letter too late (which is super confusing).