tool_urlglob: unify return codes to use CURLcode

There was a mix of GlobCode, CURLcode and ints and they were mostly
passing around CURLcode errors. This change makes the functions use only
CURLcode and removes the GlobCode type completely.
tool_operate: we prefer 'CURLcode result'
glob_next_url: make the loop count upwards

As the former contruct apparently caused a compiler warning, mentioned
in d8efde07e556c.