C3: Rename all iid fields to id

[ywarnier]

The iid fields name in C2 was a hack to move away from the c_id+id "composed primary key" in C1, authorized by MyISAM in MySQL.

That composed primary key was itself a hack to move away from the old-days structure of Claroline and the first versions of Dokeos with one database per course (where each database had its own separate documents table and separate index sequences for documents of each course).

C3 should be the right time to put some order into that and rename everything to id (simply).