fix backspace deleting login prompt

devkbd directly echos to the terminal,
but it doesn't know the line buffer length.
Instead, let devcons control the echoing.

Move kbd functions from devcons into devkbd.
Remove kbdputc from fns.h
Create consreadkbdline to reduce indentation.
Remove the now unneeded precondition in the echo function.

Backspace now deletes a UTF-8 codepoint during the login prompt.
Visual mismatch may still happen if a codepoint is not a glyph.
It's good enough for now.

Thanks,
Amavect