Replace avhttp with libcurl

Replace avhttp with libcurl

Some fixes:
1. Fixed an assumption somebody originally made in the plugin that
   std::string had contiguous storage. This is not specced behavior pre
   C++11 so we gamble by calling .reserve() which should give a far
   better guarantee.

2. Added copy to clipboard information for curl

3. Removed some openssl references in compiling.md

4. Renamed struct vars to be uppercase to match "public var" code style
   policy

Gbp-Pq: Topic fixes
Gbp-Pq: Name Replace-avhttp-with-libcurl.patch