Shrink inter-road pad size to less than the size of an Urbit page

[mcevoypeter]

2cacb2c49bbd44c82245c3910f6a78c9707ae183 introduced a fix (in the form of u3e_ward()) for the bug where the guard page resides within the pad between roads, allowing it to be "hopped" and rendered useless. A simpler alternative is to shrink the inter-road pad size to less than the size of an Urbit page, which means the pad will never be large enough to contain the entire guard page, thereby rendering it impossible for the guard page to be hopped.