Remove O_EXTRA which was only for EMX and cygwin.
Add the neovim style guide to the contributing guide.
Broken build on 32 bit: Pending TODO: Reenable assertion.

On a1a0c00589a8efc664db82be6743136bb462e08f, an assertion was disabled
because of breaking 32bit build.
This fixes that so that it now always works.
Call to list_append_dict cannot fail.

Clean up the use of list_append_dict and remove error checks.
Call to list_append_string cannot fail.

Clean up the use of list_append_string and remove error checks.