getstr() doesn't support the delete key

Curses getstr prompts are dumb, and we need to teach it how to be a sane text editor. Arm's TextInputValidator class [1] supports the left/right arrows, home, end, and esc keys. We should add support there for the delete key too.

This should be easy...

• add a line here to log keycodes
• run arm, navigate to the interpretor panel, hit the delete key
• see in the log what the keycode is
• add a handler for that keycode to delete the next character

Another minor bug is that hitting backspace when at the start of the line deletes the first character.

[1] https://gitweb.torproject.org/arm.git/blob/HEAD:/src/util/textInput.py#l12