interactive: rewrite handling user input

Split the code into two files - interactive.c and interactive-commands.c
and simplify the code.
wldbg: make wldbg_fd_callback opque struct

User doesn't need to see nor use this struct. Giving just function
that is to be called on active fd is better approach
and the code looks much better.
interactive: use different SIGINT handler

Instead of exititng wldbg on ^C, just prompt user for input.
That feels right

Signed-off-by: Marek Chalupa <mchqwerty@gmail.com>
interactive: fix debug messages
wldbg: do not write into the connection after error or exit

When some pass sets error or exit flags, do not write to the connection
anymore. This can happen for example in interactive mode after
quit command.

Signed-off-by: Marek Chalupa <mchqwerty@gmail.com>