Use g_dir_open() and friends instead of opendir() and friends.

Use g_dir_open() and friends instead of opendir() and friends.

This makes more things work on Windows, since Mingw
opendir() is kind of flakey, and breaks e.g. vcalendar or rssyl
being able to read contents of their folders.

I only left opendir() in archive plugin, since it is only found
in unused part of code (excluded by #ifdef _TEST, which is never
true for claws-mail compilations).