systemctl,man: use UNIT as the placeholder for a unit name

systemctl,man: use UNIT as the placeholder for a unit name

NAME is kind of meaningless, because everything has a name. "Unit"
makes it more obvious that a name of a unit is necessary. I was always
momentarily baffled by "set-property NAME ASSIGNMENT...", where there
are two objects (the unit and the property), and it's not clear which of
the two "NAME" is supposed to signify.