mnet: backup_user_info() is faster, and it store mnethosturl for foreign users

mnet: backup_user_info() is faster, and it store mnethosturl for foreign users

Several changes to backup_user_info() rolled up here

 - The main JOIN happens in SQL instead of PHP
 - We now also join with mnet_host to get the wwwroot
 - As part of the join, all the variables appear in $user -- $user_data is gone
 - If the mnethostid is non-local we include MNETHOSTURL for that user

With this, backup_user_info() should be significantly faster for large
sets of users, though I haven't benchmarked it.