hooks/fancy: print_stack_update: adjust PRINT_BUSY_STACK_LENGTH relatively and export using `export`, not `declare -x`.

- exporting with `declare -x` doesn't seem to work;
- adjustments should be relative because PRINT_BUSY_STACK itself is not exported,
  so nested bash instances can't observe the full stack and get correct length.
hooks/fancy: handle_output_create: export PRINT_STDERR and PRINT_STDOUT.
hooks/fancy: add output_force_term: function to make command write to real terminal.

Interactive commands may be wrapped using this function so that output handler
will not break them.

The function also "notifies" the helper (using RI+LF sequence) to avoid merging BUSY and DONE
lines in assumption that the executed command did actually write something to the terminal.