Move ip requesting code into a separate file and make hashing configurable at build time.

New code tries to guess a per-visitor IP address and uses
that (or its hash) to tell users apart.
This patch also adds a TAWASHI_WITH_IP_LOGGING cmake option
to enable or disable logging the IP address of your visitors
in various places (just one right now but don't assume things
to remain this way).
Also added a couple new CGI environment variables.

src/tawashi_implem/CMakeLists.txt

@@ -28,6 +28,7 @@ add_library(${PROJECT_NAME} STATIC
 	tawashi_exception.cpp
 	http_header.cpp
 	quick_submit_paste_response.cpp
+	ip_utils.cpp
 )
 
 target_include_directories(${PROJECT_NAME}