Backport several changes from my Debian packaging work

[mernisse]

• Makefile now honors CFLAGS and DESTDIR from the debhelper build system.
• modify anetWrite() to handle EAGAIN so I can send data larger than SO_SNDBUF if needed.
• Modify the HTTP server to use anetWrite() to prevent short writes while sending content.
• Modify the HTTP server and script.js to pass --lat and --lon arguments to the web interface via an AJAX call to config.json.
• Set the correct HTTP status code if we have problems serving a file.

[bovine]

There are some merge conflicts that prevent this from applying cleanly to the current codebase.

[mernisse]

I updated my changes to match the current codebase.