In D7610 I've broken mkdir in krarc. This patch should:

• fix mkdir for krarc (by actually calling KIO::mkdir when only one directory is being created)
• fix mkpath for krarc (by reporting mkdir success with paths like krarc:/home/)

Other changes was just tiny code optimizations.

Any criticism is appreciated.

• create a directory inside archive
• create multiple directories inside archive (e.g. "aaa/bbb/ccc")