Make "/tools/cron.php" script act as Front-End in regards to link building

By default cron (/tools/cron.php script) does it's job silently without sending any e-mail to anybody.

However if add a Scheduled Task that would, for example, send out latest added links on a website, then you'll notice soon enough that there are several problems in these links:

• links look like http://localhost/ (fixed in #0000849)
• links look like http://www.website.tld/tools/index.html (fixed in #001005)
• links look like http://www.website.tld/admin/tools/cron.php?env=.... (when mod-rewrite is enabled)

This all is happening because of ADMIN constant being defined in cron.php script. Maybe it's defined to skip most of front-end specific code, which also includes mod-rewrite link building.

I'm proposing to remove this constant now and in code, that really isn't needed when cron is executed from PHP CLI place if with check for CRON constant it defined all the time.