Remove a couple of warnings

cut/cut.rs

@@ -14,7 +14,6 @@
 extern crate getopts;
 extern crate libc;
 
-use std::os;
 use std::io::{File, BufferedWriter, BufferedReader, stdin, print};
 use getopts::{optopt, optflag, getopts, usage};
 

nohup/nohup.rs

@@ -35,7 +35,10 @@ extern {
 
 #[cfg(target_os = "macos")]
 fn rewind_stdout<T: std::rt::rtio::RtioFileStream>(s: &mut T) {
-    s.seek(0, std::rt::rtio::SeekEnd);
+    match s.seek(0, std::rt::rtio::SeekEnd) {
+        Ok(_) => {}
+        Err(f) => crash!(1, "{}", f.detail.unwrap())
+    }
 }
 
 #[cfg(target_os = "linux")]