Proper unit usage

New patch for proper unit-usage in core.
Here is what the patch does:
- it makes use of drupal api's to display the part of the url in front of path textfields (path.module/system.module);
- it gives us consistent textfield sizes between upload/user module for specifying resolutions and file sizes;
- have user.module make use of the '#field_(prefix|suffix)' where needed;
- did some small part of more consistent code markup for the appropriatary functions;

Please review and apply...