[usrloc module]: added a new 'attr' column in the location table, used to
store additional registration-related information

[registrar module]: new avp used to load/store the 'attr' column
information in the location table. Its content is read and stored in db and/or
memory when doing a 'save' operation, and it is populated after doing a 'lookup'.

When doing call forking, the avp will hold a value for each branch. The 'attr'
value of a branch is accessed using $T_branch_idx.