libfuse: allow disabling adaptive readdirplus

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).