Add FUSE_READDIRPLUS support

[asomers]

Currently, an ls -l command must issue at least one FUSE_READDIR operation followed by many FUSE_LOOKUP operations. Each of the FUSE_LOOKUP operations requires re-reading the directory's inode and at least some of its directory blocks. That's a lot of redundant disk reads. We should consider adding support for the FUSE_READDIRPLUS operation, which combines READDIR and LOOKUP. That would significantly reduce the number of disk reads. OTOH, #107 would also reduce the number of disk reads. FUSE_READDIRPLUS may not be necessary after completing #107 .