HTTP refactoring/cleanup

The following set of patches cleans up http-fetch and http-push.

- Common HTTP code has been moved to http.c/http.h. While curl-specific code still exists in both http-fetch and http-push, it is a step in the direction of allowing use of a different HTTP request library if someone is so inclined.

- Object request functions and data have been renamed to keep clear what types of requests are being processed, which will make it easier to process additional types of requests in parallel (eg. starting pack downloads immediately when an object 404s)

- The reliability and performance of http-fetch has been improved when built in an environment without curl multi support.

- XML parsing in http-push has been improved, which will make it easier to add functionality to update remote server info.

For a successful technology, reality must take precedence over public
relations, for nature cannot be fooled.