Return values added to Cron API functions to indicate outcome:
* `wp_schedule_single_event()`, `wp_schedule_event()`, `wp_reschedule_event()` and `wp_unschedule_event()`: boolean indicating success or failure,
* `wp_clear_scheduled_hook()`: integer indicating number of jobs cleared (zero or more), `false` if one or more jobs fail to clear,
* `wp_unschedule_hook()`: integer indicating number of jobs cleared (zero or more), `false` if the jobs fail to clear,
* `spawn_cron()`: boolean indicating whether job spawned,
* `wp_cron()`: integer indicating number of jobs spawned (zero or more), `false` if one or more jobs fail to spawned,
* `_set_cron_array()`: boolean outcome of `update_option()`.
Props evansolomon, jrf, peterwilsoncc, pento for code review.
Fixes#21072.
git-svn-id: https://develop.svn.wordpress.org/trunk@43050 602fd350-edb4-49c9-b593-d223f7449a82