libfuse: rename "readdir_plus" to "readdirplus"

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.
libfuse: implement readdirplus for high-level API

Existing FUSE filesystems (e.g. fusedav) with a complete
"filler" function for their existing readdir implementation
can simply reuse their readdir callback as the readdirplus
callback.

Tested against fuse.git

Changes since v1:
* moved readdirplus to the bottom of struct fuse_operations to
  increase the likelyhood of ABI compatibility
* correctly handles NULL stat buffers in readdirplus if the
  user FS is unable to supply stat information for some dentries.
* consistently uses readdirplus offsets in case of adaptive
  readdirplus usage.  This requires my RFC posted to:
  http://mid.gmane.org/20130205031202.GA9062@dcvr.yhbt.net

Changes since v2:
* consistent terminology for "readdirplus" (no underscores)
libfuse: allow disabling adaptive readdirplus

This switches the -o no_readdirplus option to a tristate
string: -o readdirplus=(yes|no|auto)

Telling the kernel to always use readdirplus is beneficial to
filesystems (e.g. GlusterFS) where the cost to perform readdir
and readdirplus are identical.

The default remains "auto" (if supported).