- Removed some code duplication
- Ctrl+D is fixed and now works as it should (exit if user didn't
write anything, ignore otherwise)
- Ctrl+L clears the terminal screen
- readline function now uses raw terminal input mode (if in
interactive session)
- readline function supports cursor movement to the left and to the
right and inline editing (also unicode support)
- if arsh has a PS1 env var, it will use it to display prompt. It
supports esc sequences, \u, \w
- if '~' is used, it will be expanded to $HOME env var value
- if PS1 env var uses \w, and $PWD is somewhere inside $HOME, then it
will show '~' in prompt instead of a long path
- arsh sets $SHELL env var on startup
