getstr's insert mode (Ctrl-V) indicator and edit/line modes. Global Hot Keys
(e.g. Ctrl-T) would do weird stuff like return the cursor to the end of the
line or redraw the input string multiple times.
So as part of this fix:
- backspasce() now removed the last count characters from the save-line buffer
  (it is a destructive backspace afterall).
- outchar('\b') (a non-destructive backspace) adds the \b to the save-line buf
- restoreline() now always redraws the current insert/overwrite indicator in
  the upper right-most corner fo the screen
- insert_indicator() now uses functions that aren't line-saved.

An unrelated change included in this comment: center() now sends a CR before
the cursor movement and centered-text, for times when the current column is
*not* already at 0. Very useful for use with @-codes (e.g. display some pattern
or "graphics" and then center some text over it).