Enable/disable scan_and_update_registry() based on commandline switch or environment variable. Fixes #520468.

Original commit message from CVS:
patch by: Jason Zhao <e3423c@motorola.com>
* docs/gst/running.xml:
* gst/gst.c:
Enable/disable scan_and_update_registry() based on commandline switch
or environment variable. Fixes #520468.
* ChangeLog:
Fix typo in my previous commit.