api: add nvim_create_buf to create a new empty buffer.

Loading existing files into a buffer is non-trivial and requires a window.
Creating an unnamed emtpy buffer is trivial and safe though, thus worth a
special case.

Change nvim_buf_set_option to use aucmd_prepbuf. This is necessary
to allow some options to be set on a not yet displayed buffer, such
as 'buftype' option.

vim-patch:7.4.1858: Add BLN_NEW to enforce buflist_new creating new buffer

src/nvim/api/vim.c

@@ -955,6 +955,25 @@ void nvim_set_current_win(Window window, Error *err)
   }
 }
 
+/// Create new empty buffer
+///
+/// @param listed whether the buffer should be listed
+/// @param[out] err Error details, if any
+/// @return the buffer handle or 0 when error
+Buffer nvim_create_buf(Boolean listed, Error *err)
+  FUNC_API_SINCE(6)
+{
+  try_start();
+  Buffer buffer = buflist_add(NULL,
+                              BLN_NOOPT | BLN_NEW | (listed ? BLN_LISTED : 0));
+  if (!try_end(err) && buffer == 0) {
+    api_set_error(err,
+                  kErrorTypeException,
+                  "Failed to create buffer");
+  }
+  return buffer;
+}
+
 /// Gets the current list of tabpage handles.
 ///
 /// @return List of tabpage handles