Fix shell crash when in switch command mode

The user_drv process can crash if ctrl-y is pressed when in switch command mode (ctrl-g).
This is due to switch mode using edlin to parse the input.
And edlin uses the process dictionary to store the kill_buffer.
However the kill_buffer is undefined if not initialized by edlin:init/0
which makes the user_drv process crash since edlin is calling list:reverse/2 with undefined.

Fix calles edlin:init/0 once before server_loop is entered.