Bazel client: platform-specific {Read,Write}File

Move blaze::ReadFile and blaze::WriteFile to
file.h and file_platform.h (thus into the
blaze_util namespace), and update references.

This allows us to implement these methods in a
platform-specific way.

Also move UnlinkPath.

See https://github.com/bazelbuild/bazel/issues/2107

--
MOS_MIGRATED_REVID=140328273
diff --git a/src/main/cpp/option_processor.cc b/src/main/cpp/option_processor.cc
index e3224a2..680e2e3 100644
--- a/src/main/cpp/option_processor.cc
+++ b/src/main/cpp/option_processor.cc
@@ -74,7 +74,7 @@
   string filename(filename_ref);  // file
   BAZEL_LOG(INFO) << "Parsing the RcFile " << filename;
   string contents;
-  if (!ReadFile(filename, &contents)) {
+  if (!blaze_util::ReadFile(filename, &contents)) {
     // We checked for file readability before, so this is unexpected.
     blaze_util::StringPrintf(error,
         "Unexpected error reading .blazerc file '%s'", filename.c_str());