Rename status, cron and rsync commands to 'core-*' to avoid conflicts with other modules.

In addition to the enclosed patch, the file commands/core/rsync.inc must be renamed to commands/core/rsync.core.inc.

As previously discussed, I did not rename 'help'. Also, I did not rename updatedb to core-updatedb because I sort of wondered if it shouldn't be moved and renamed to pm-updatedb. It could stay as it is, or I could rename it to core-* or I could move it to pm-* per your preference.

As a bonus, I also added a feature to show-invoke that puts a [*] next to available function hooks that are actually implemented, which is quite a useful enhancement.