sh: Keep output buffer across builtins.

Allocating and deallocating repeatedly the 1024-byte buffer for stdout from
builtins costs CPU time for little or no benefit.

A simple loop containing builtins that write to a file descriptor, such as
  i=0; while [ "$i" -lt 1000000 ]; do printf .; i=$((i+1)); done >/dev/null
is over 10% faster in a simple benchmark on an amd64 virtual machine.

bin/sh/output.c

@@ -254,14 +254,7 @@ flushout(struct output *dest)
 void
 freestdout(void)
 {
-	INTOFF;
-	if (output.buf) {
-		ckfree(output.buf);
-		output.nextc = NULL;
-		output.buf = NULL;
-		output.bufend = NULL;
-	}
-	INTON;
+	output.nextc = output.buf;
 }