synced_folders/rsync: Install `rsync` on guest if needed

Add new `rsync_installed` and `rsync_install` guest capabilities
to detect and install `rsync`.

Also copy `rsync_pre` capability to all Unix guests.
Merge branch 'rsync_install' of https://github.com/tmatilai/vagrant into tmatilai-rsync_install

Conflicts:
	plugins/guests/freebsd/plugin.rb
	plugins/guests/netbsd/plugin.rb
	plugins/guests/openbsd/plugin.rb