core: Use mmap() with O_NOATIME if merely reading archive-z2 entries

core: Use mmap() with O_NOATIME if merely reading archive-z2 entries

Previously we'd open(path, O_NOATIME) and do a series of small read()
calls to just parse the header.  I think this will trigger kernel readahead
into the compressed portion, but we don't care about that.

This should be more efficient.