REORG: http: move error codes production and processing to http.c

REORG: http: move error codes production and processing to http.c

These error codes and messages are agnostic to the version, even if
they are represented as HTTP/1.0 messages. Ultimately they will have
to be transformed into internal HTTP messages to be used everywhere.

The HTTP/1.1 100 Continue message was turned to an IST and the local
copy in the Lua code was removed.