One who seeks the truth...especially in the sense of resisting a possible cover up by the government.
Oh, now, governments never lie, though, right? We can trust them. Uh huh.
To carry your ANALolgy further:
A truther seeks the truth. Nothing complicated about that.
A birther questions Obama's birthplace. THAT is semantics.
But a truther seeks the truth...no matter what the cost. That is NOT semantics.
Time to INVESTIGATE the investigators of 9/11. PROSECUTE the prosecutors. EXPOSE the cover-up.