[RFC] Add `feedkeys` API function part 2

Cleaning up after #916

- I've split the f_feedkeys() function from eval.c into a reusable feedkeys()
- Now the API vim_feedkeys should work just like the vimscript feedkeys(String keys, String mode)
- Added new vim_remote_send(String) API for compatibility w/ Vim --remote-send

`flags` could be `NULL` at this point (if `argvars[1].v_type == VAR_UNKNOWN)`. `cstr_as_string` would not like this.