Remove "hack" with no input line being NULL

It caused more bugs and sloppy code than being useful.
Introduce replace_wstring() to modes/cmdline.c
Move dependent lines in cmd_ctrl_c() closer
Introduce modes/cmdline:is_input_line_empty()

Checks whether input line is empty.

For better readability.
Merge branch 'do-not-use-null-buf-in-cmdline-mode'

Get rid of old "trick" with NULL line denoting an empty line.