Fix border-radius and :focus styles for custom form controls

Created by: claviska

1. The custom select menu doesn't honor the $enable-rounded flag. It should also set the border-radius property using $input-border-radius for consistency.
2. When $enable-rounded is false, a 5px border-radius persists on the custom select menu control. It appears to be from Chrome's user agent stylesheet. This can't be reset using the border-radius mixin because it doesn't execute when $enable-rounded is false. ce77c09f fixes both these issues.
3. Input group addons use $border-radius instead of $input-border-radius, making them inconsistent with form controls when the default value is changed. 7430a263 fixes this.
4. Custom select menus, file inputs, checkboxes, and radios have hard-coded colors for their focus states. 463d83e8, 5f57e41f, and fe47d5cb let them use $input-border-focus variable to stay consistent with other form controls.
5. The focus state on file inputs is not visible when $enable-rounded is false. ecc75f93 fixes this in the same manner that custom checkboxes/radios force the focus state.
6. Custom file inputs use hardcoded values. 9fcfdf9b and e940d607 makes them use variables to stay consistent with other form controls.
7. The box-shadow size for custom select menus is inconsistent with other form controls. Using the form-control-focus mixin in a0757d11 fixes this.

Sorry for all the little updates—I'm working on something with v4-dev and finding/fixing as I go.