Keep STDOUT/STDERR the right mode. Fixes #422

If you reopen STDOUT with an IO object that is "w+", then later reopen
it again as just "w", you get an exception:

`reopen': /dev/null can't change access mode from "w+" to "w" (ArgumentError)

lib/puma/daemon_ext.rb

@@ -13,10 +13,11 @@ module Process
     Dir.chdir "/" unless nochdir     # Release old working directory.
 
     if !noclose
-      null = File.open "/dev/null", "w+"
-      STDIN.reopen null
-      STDOUT.reopen null
-      STDERR.reopen null
+      STDIN.reopen File.open("/dev/null", "r")
+
+      null_out = File.open "/dev/null", "w"
+      STDOUT.reopen null_out
+      STDERR.reopen null_out
     end
 
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