Port who(1) to utmpx.

Port who(1) to utmpx.

(Un)fortunately there is no standardized interface to switch between
utmp database files, so we must call ulog_setutxfile() here.

I'm also changing the column widths to magic numbers here. Display
layout should in this case not be derived from structure fields sizes.
Because I don't want struct utmpx ever to become too small, the fields
are too big to reserve all the space.