Split f_feedkeys into a separate feedkeys function

- This commit separates the feedkeys logic from the vimscript
  checks in preparation for using feedkeys() in C code.
Make vim_feedkeys a simple wrapper around feedkeys

- To clean up the mix between feedkeys and replace_termcodes
  the vim_feedkeys API functions now does the same thing as the
  vimscript feedkeys() function
Add --remote-send compatibility API

- Vim has --remote-send to push keys into a remote Vim, this function
  backports the remote-send handler as a Neovim API
- vim-patch:0