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.osx b/Makefile.osx
index 76da19e..31a7f99 100644
--- a/Makefile.osx
+++ b/Makefile.osx
@@ -110,7 +110,7 @@
        src/peers.o src/stream_interface.o src/dumpstats.o src/proto_tcp.o \
        src/session.o src/hdr_idx.o src/ev_select.o src/signal.o \
        src/lb_chash.o src/lb_fwlc.o src/lb_fwrr.o src/lb_map.o src/lb_fas.o \
-       src/ev_poll.o \
+       src/ev_poll.o src/connection.o \
        src/arg.o src/acl.o src/memory.o src/freq_ctr.o \
        src/auth.o src/stick_table.o src/sample.o