MINOR: connection: add a handler for fd-based connections

This connection handler will be used as an I/O handler for events
detected on a file descriptor. It is not used yet.
diff --git a/Makefile b/Makefile
index 7cc1708..f73c8da 100644
--- a/Makefile
+++ b/Makefile
@@ -543,7 +543,7 @@
        src/uri_auth.o src/standard.o src/buffers.o src/log.o src/task.o \
        src/time.o src/fd.o src/pipe.o src/regex.o src/cfgparse.o src/server.o \
        src/checks.o src/queue.o src/frontend.o src/proxy.o src/peers.o \
-       src/arg.o src/stick_table.o src/proto_uxst.o \
+       src/arg.o src/stick_table.o src/proto_uxst.o src/connection.o \
        src/proto_http.o src/sock_raw.o src/appsession.o src/backend.o \
        src/lb_chash.o src/lb_fwlc.o src/lb_fwrr.o src/lb_map.o src/lb_fas.o \
        src/stream_interface.o src/dumpstats.o src/proto_tcp.o \