AU: MultiHttpFetcher cleanup/rewrite

This is the first of many CLs to cleanup/refactor/unfork the
HttpFetcher classes.

This CL changes MultiHttpFetcher to MultiRangeHTTPFetcher, makes it
work with a single base fetcher, and un-templatizes it.

Also, fix a (new?) bug in SConstruct w/ setting CCFLAGS.

TEST=unittests, tested an interrupted/resumed update on device.
BUG=10395

Review URL: http://codereview.chromium.org/5835004

Change-Id: I8422358a6d425233987dd799c5ee7c87135d85fd
diff --git a/libcurl_http_fetcher.cc b/libcurl_http_fetcher.cc
index cdc7523..16d0239 100644
--- a/libcurl_http_fetcher.cc
+++ b/libcurl_http_fetcher.cc
@@ -156,6 +156,7 @@
   retry_count_ = 0;
   no_network_retry_count_ = 0;
   http_response_code_ = 0;
+  terminate_requested_ = false;
   ResolveProxiesForUrl(url);
   ResumeTransfer(url);
   CurlPerformOnce();