Document that d_ino == 0 represents an "unknown" inode number

[sunfishcode]

As discussed in #65, to allow for platforms where the obvious readdir implementation doesn't provide an inode number, allow d_ino to be zero, indicating an unknown number. libc can implement userspace d_ino by doing an fstatat call.

[sunfishcode]

This is obsoleted by #72, which fixes this by making the inode field optional rather than special-casing zero, as zero is a valid inode number on some filesystems.