REORG: httpclient/lua: move the lua httpclient code to http_client.c

REORG: httpclient/lua: move the lua httpclient code to http_client.c

Move the lua httpclient code from hlua.c to http_client.c

The code is almost the same but the registering of the class which is
done in hlua_http_client_init_state(), from REGISTER_HLUA_STATE_INIT()

check_args() calls have been replaced by hlua_check_args().

hlua_httpclient_destroy_all() is exported so it can be called in hlua.c.
hlua_httpclient_table_to_hdrs() is made static.