When an app in a FUSE mount calls getdents(2), go-fuse receives
READDIR[PLUS] and calls the filesystem's OpenDir function that
returns []DirEntry.

The data returned from getdents(2) contains an inode number for
each directory entry, "d_ino". Until now, struct DirEntry had no
corresponding field and the value passed to the kernel was always
FUSE_UNKNOWN_INO = 0xffffffff

This broke apps that actually look at the d_ino field, like
"find -inum".

This commit adds the "Ino" filed to struct DirEntry. If the field
is not set by the filesystem, it is set to FUSE_UNKNOWN_INO,
as before. Otherwise it is left alone and passed to the kernel.

loopbackFileSystem's OpenDir function is extended to set the inode
number. A test verifies that the returned inode number is sane.

Fixes https://github.com/hanwen/go-fuse/issues/175