AU: clean up logs

- don't log for each operation when doing a delta update (there are
  thousands)
- Fix subprocess to only log if there is output
- Fix subprocess to say 'subprocess' rather than 'postinst', since the
  sub process is not always postinst :)

BUG=5928
TEST=unit tests

Review URL: http://codereview.chromium.org/3119031
diff --git a/subprocess.cc b/subprocess.cc
index c319a22..99605dd 100755
--- a/subprocess.cc
+++ b/subprocess.cc
@@ -139,10 +139,10 @@
   FreeArgv(argv.get());
   if (err)
     LOG(INFO) << "err is: " << err->code << ", " << err->message;
-  if (child_stdout)
-    LOG(INFO) << "Postinst stdout:" << child_stdout;
-  if (child_stderr)
-    LOG(INFO) << "Postinst stderr:" << child_stderr;
+  if (child_stdout && strlen(child_stdout))
+    LOG(INFO) << "Subprocess stdout:" << child_stdout;
+  if (child_stderr && strlen(child_stderr))
+    LOG(INFO) << "Subprocess stderr:" << child_stderr;
   return success;
 }