[update_engine] Fix file creation mask to be 0600

In addition to changing the write() call in utils.cc, I also set the umask
of the process to be 177 -- meaning that files will be AT MOST -rw-------

I do this _after_ we initialize logging, so that we don't create log files
with unnecessarily restrictive permissions.

BUG=chromium-os:6581
TEST=Unit tests,

Change-Id: Id6b805a1524af391755bc1df69ec0f6c382154c2

[update_engine] Ensure that created files have restrictive permissions

Change-Id: If93e043465083f7c48619d0e7163dd73f8c46090

Review URL: http://codereview.chromium.org/3495002
diff --git a/extent_writer.h b/extent_writer.h
index 5eda364..1bb6f4a 100644
--- a/extent_writer.h
+++ b/extent_writer.h
@@ -66,7 +66,7 @@
 
  private:
   int fd_;
-  
+
   size_t block_size_;
   // Bytes written into next_extent_index_ thus far
   uint64_t extent_bytes_written_;