coding standards fixes. Improved some admin setting page form descriptions and messages.

added a function to return the default account reminder email subject and body text to be
used in the settings page and for mail messages when there is no user defined default.
collapsed two if clauses into a single OR clause in account_reminder_cron().
#486308 Add a configurable cron limit for how many messages to process on every cron run.
change placeholder for watchdog message to %.