Update doc-present to use changed variable names of doc-view

- doc-view seems to use variables with a double dash
- doc-view--current-cache-dir instead of doc-view-current-cache-dir
- doc-view--current-converter-processes instead of
  doc-view-current-converter-processes

I added two checks if a variable with the old name is bound and if it is
void, it uses the new variable names.