create the directory for a directory entry with no trailing slash

[jmestwa-coder]

A directory entry can be spelled with the DOS directory bit instead of a trailing slash. Nothing creates that directory, so the mode restore in zip_archive_extract lands on whatever already occupies the path:

• keep.txt carrying the dir bit and mode 0777 turns an existing 0600 keep.txt into 0777 without a byte of it being written, and zip_extract returns 0
• with nothing at that path the chmod fails ENOENT and the extraction aborts with ZIP_ENOPERM, and the directory is never created
• zip_mkpath only creates components that end at a separator, which is why the trailing-slash spelling is covered and this one is not

Creates the directory for that spelling, and returns ZIP_ECHKDIR when the path is held by something that is not a directory, the same as zip_mkpath does. zip_entry_fread is unaffected, it rejects directory entries before its own chmod. Two tests under test_extract cover both halves; they fail on master with 0 and ZIP_ENOPERM.