Run remote rsync as root to guarantee that rsync can write to guestpath.
This obviates the need to chown the guestpath to the SSH user prior to
sync.
This brings a substantial speedup (2x on a moderately-sized shared
folder) and properly triggers filesystem notifications on only the files
changed by a given sync.
rsync should *always* be pre-installed in SmartOS (global and local zones), as
it's part of the kernel. Previous commits incorrectly attributed #rsync_install
to running rsync, rather than to installing the rsync command.
Teemu Matilainen
Mitchell Hashimoto
Nikhil Benesch
Eric Saxby