parent: Unroll os.close() loop in first stage

SSH command size: 469 (-11 bytes)
Preamble size: 8946 (no change)

Although the source is longer, the _compressed_ length is reduced.

mitogen/parent.py

@@ -330,7 +330,10 @@ class Stream(mitogen.core.Stream):
             os.dup2(0,100)
             os.dup2(R,0)
             os.dup2(r,101)
-            for f in R,r,W,w:os.close(f)
+            os.close(R)
+            os.close(r)
+            os.close(W)
+            os.close(w)
             os.environ['ARGV0']=e=sys.executable
             os.execv(e,['mitogen:CONTEXT_NAME'])
         os.write(1,'EC0\n')