Add libxo(3) support to last(1).

[philip]

Summary

Adds libxo(3) support to last(1).

Test Plan

• Diff confirms that output without --libxo is unchanged
• Output with --libxo looks sane

[kp]

usr.bin/last/last.c

121

I wonder if it's worth putting this in an atexit(3) callback. There are at least 4 calls to xo_finish() spread around the code, and it's easy to forget one, now or later if more are added.

[philip]

• last: use xo_finish_atexit wrapper