API: nvim_put: always PUT_CURSEND

Fixes strange behavior where sometimes the buffer contents of a series
of paste chunks (vim._paste) would be out-of-order.

Now the tui_spec.lua screen-tests are much more reliable. But they still
sometimes fail because of off-by-one cursor (caused by "typeahead race"
resulting in wrong mode; fixed later in this patch-series).

test/functional/api/vim_spec.lua

@@ -382,6 +382,7 @@ describe('API', function()
         line 1
         line 2
         line 3]])
+      command('1')
       -- blockwise
       nvim('put', {'AA','BB'}, 'b', false)
       expect([[