Avoid mkdir /etc/rc.d when installing pbulk tools

[jlmuir]

When installing the pbulk tools, rsync is installed, and even though the rsyncd system start-up script is not installed, the pkgsrc package install logic tries to create /etc/rc.d, and on macOS Sierra, this results in a mkdir error message like the following:

  # pkg_comp bootstrap
  ...
  ===> Binary install for pbulk-0.65
  => Installing pbulk-0.65 from /pkg_comp/packages/pbulk/All
  mkdir: /etc/rc.d: Read-only file system

Avoid the mkdir /etc/rc.d attempt by setting RCD_SCRIPTS_DIR to a path under the pbulk tree which is writable, so no error message is emitted.