Write ask_user(). Integrate with create_bkp().

Print warnings and other info to stderr, because stdout is used for return
values.
Add generic prompt for errors and warnings. Add rm_bkp(). Call editor.

Also:
    - Change 'exit' to 'return' in functions.
    - Exit if backup creator's process still running,
    - Create backup file and proceed regardless of whether previous backups
      were found or not. If they was, do this after asking user about each of
      them.
Finally fix filtering of duplicate answers in ask_user().

Now i do not reorder answers in possible asnwers list during filtering out
duplicate entries.
Ask user what to do after editor is finished.

Also:
    - Redirect `cp` and `rm` output to stderr in create_bkp().
Retab with 'expandtab' enabled.
Fix broken changes confirmatiom after editor run. Remove rm_bkp().

Do not use ask_user(), when removing previous backups: just `rm -i`.
Do not use rm_bkp(): just `rm -i`.
Finally move FDs for normal writing to stdout in functions returning result through stdout.

This is really simplier, then redirecting each command, who want to write
sometinhg, to stderr.
Ensure, that i backup and restore file pointed by symlink.
Move creator's PID check to is_bkp_creator_alive() .

Strictly speaking, this is the same. But now i can say, that you can't assume,
that is_bkp_creator_alive() function returns PID: it may be anything else. The
only proposition, which is true: if it returns something, creator still alive.

So, this is just to ensure smoother (possible) transition to new backup file
name format, which does not include creator's PID at all.