Don't allow users to run Set Membership Reminder Dates scheduled job automatically

"Set Membership Reminder Dates" is a job that should only be run ONCE manually when a site first implements scheduled renewal reminders AND already has existing memberships that need a reminder date set. Currently, admins can ENABLE this job and cause it to run as part of a consolidate job run (using 'execute' action). This can cause constituents to get repeated renewal reminders if the "Update Membership Statuses and Send Renewal Reminders" job is also run.

Details here:
http://forum.civicrm.org/index.php/topic,23815.msg100076.html#msg100076

Solution is to force this job status to "Disabled" as part of the 4.1.2 upgrade, AND to add special logic in the Manage Schedule Jobs page which prevents administrators from ever enabling this job. In this way, the job can ONLY be run "manually" - either by explicitly calling it from command line (cli.php), OR by using the "Execute Now" action in the UI for this job only.

The "Update Greetings and Addressees" job is also intended for "special use" - not to be run regularly. The same approach will be done for that job as well (although the consequences of running this job multiple times is generally not serious - there is the possibility of unintended data modification or loss).