libfuse: add readdirplus support in fuse_lowlevel_ops

libfuse: add readdirplus support in fuse_lowlevel_ops

This patch implements readdirplus support in FUSE usersapce. It adds
a new fuse lowlevel operations fuse_lowleve_ops::readdir_plus,
corespoding mount options and helper functions to maintain buffer.

[From: Eric Wong <normalperson@yhbt.net>]

This makes our terminology consistent with NFS and
our kernel module, as well as reducing user/developer
confusion in the command-line.

Note: I'm keeping "fuse_add_direntry_plus" since that is
less standardized in its use than "readdirplus" for now.

Signed-off-by: Feng Shuo <steve.shuo.feng@gmail.com>