User module patch by Kjartan:

- Optimized user_external_load(), only need to fetch the data once.
- Removed unused functions:
  user_get()
  user_set()
  theme_menu_list()
- Fixed user_validate_name() to not accept \.
- Modified user_validate_name() to use alnum, which varies depending on
  the system locale.
- Optimized user_validate_name() to not use unnecessary regular
  expressions.
- Optimized user_access() by using less logic to fetch permissions and
  cache.
- Removed uncessary logic from user_deny().
- Fixed user_fields() to properly cache $fields.
- Optimized user_set_authmaps() to not perform unnecessary queries.
- Removed return value from user_set_authmaps().