Let w(1) use utmpx.

We don't have UT_*SIZE anymore. One of the reasons for that is because
all strings are null terminated, there is no need for apps to copy
strings out of the utmpx structure. This means we can define W_DISP*SIZE
lengths for all columns.

While there, adjust the sizes a little. Steal some bytes from the
username column, while extending the hostname column quite a bit.