Several improvements to tinyos-mirror.sh

Several improvements to tinyos-mirror.sh
* Do not run a default operation if no --option flag set
* No longer replicate tinyos-1.x by default.  We don't need it.
* Smarten up rsync so that it only replicates those parts of the remote CVS
  that are relevant to the modules provided on the command line, or the
  default modules.
* Replace occurrences of $CVSBASE/tinyos with $CVSROOT.
* Put rotation of multiple remote CVS (hard-linked) backups into its own
  function so it can in the future be optionally be disabled.
* Make the locks_present work on a per CVS module basis, so one module won't
  be prevented an import because of a lock in a different module.
* Add a .git/description to the repositories, for a description that will show
  up in gitweb.
* Add an env_ok function to test for the existence of the base directories and
  their permissions.