Verbose cron backport

With this patch of includes/common.inc cron tells you something like this:

Cron run completed: 452.03 ms. (filter:16, node:8, search:4, statistics:0, watchdog:28, backup_migrate:0, cram:8, eventrepeat:32, image:16, job_queue:184.01, persistent_login:8, simplenews:40, subscriptions_mail:84.01, update_status:0, workflow:4, cclinks:16, states:0, calendar_ical:4, subscriptions_content:0, notify:0)

also,
// If you want to log start/stop messages for each module to the watchdog,
// then add the following line to your settings.php
//
// $conf = array ('cron_debug' => 1);

This is a backport of a patch in #131536: Make cron watchdog more granular and informative with (imho) significant improvements.
The only change is the different signature of the watchdog function.
The settings.php changes are not included in this patch.