Change the terminology from "Moodle Network" to "MNet"

As of Moodle 1.9.2, the English language pack and documentation still talks about "Moodle Network", "Moodle Networking" etc. But the concept of Moodle network services is not limited to connecting Moodle servers only - e.g. Mahara can talk to Moodle pretty well.
This is a proposal to call our networking framework as "MNet" (maybe with some nice logo?) and modify all current strings accordingly (mainly mnet.php).

Discussed at http://moodle.org/mod/forum/discuss.php?d=128584