Change all relevant occurrences of ``rsync'' and ``$rsync_bin'' to the

Change all relevant occurrences of ``rsync'' and ``$rsync_bin'' to the
canonical form ``$RSYNC'' (set in testsuite/rsync.fns). This prevents any
stray rsync binaries in the user's PATH from being picked up by the test
scripts and ensures that the newly built rsync binary is used always.