more clippy fixes

use is_empty() instead of len comparison (clippy::len_zero)
use if let instead of while let loop that never loops (clippy::never_loop)
remove redundant returns (clippy::needless_return)
remove redundant closures (clippy::redundant_closure)
use if let instead of match and wildcard pattern (clippy::single_match)
don't repeat field names redundantly (clippy::redundant_field_names)

src/libstd/sys/unix/process/process_unix.rs

@@ -72,7 +72,7 @@ impl Command {
             }
         };
 
-        let mut p = Process { pid: pid, status: None };
+        let mut p = Process { pid, status: None };
         drop(output);
         let mut bytes = [0; 8];