MDL-43677 dml: Make debugging about invalid LIMIT params more picky

We will no longer silently convert empty strings/-1 to 0 in limit
params.
MDL-43677 lib: avoid using invalid invalid LIMIT params

This commit changes the following library functions so they now
default to limit params of 0 (meaning no limit applies).

* build_mnet_logs_array
* build_logs_array
* get_logs
* get_role_users
* get_users_by_capability
* tag_find_records
* tag_print_tagged_users_table
* completion_info::get_tracked_users