Auto merge of #1096 - jmmv:truncate, r=alexcrichton
Auto merge of #1096 - jmmv:truncate, r=alexcrichton

Add truncate(2) to Unix platforms

rust-lang/libc,rust-lang-nursery/libc,rust-lang/libc,rust-lang/libc,rust-lang-nursery/libc,rust-lang-nursery/libc