[MINOR] replace client_retnclose() with stream_int_retnclose()

This makes more sense to return a message to a stream interface
than to a session.

senddata.{c,h} have been removed.
diff --git a/Makefile b/Makefile
index 1cdeac1..1f940d3 100644
--- a/Makefile
+++ b/Makefile
@@ -452,7 +452,7 @@
        src/time.o src/fd.o src/regex.o src/cfgparse.o src/server.o \
        src/checks.o src/queue.o src/client.o src/proxy.o src/proto_uxst.o \
        src/proto_http.o src/stream_sock.o src/appsession.o src/backend.o \
-       src/stream_interface.o src/senddata.o src/dumpstats.o src/proto_tcp.o \
+       src/stream_interface.o src/dumpstats.o src/proto_tcp.o \
        src/session.o src/hdr_idx.o src/ev_select.o \
        src/acl.o src/memory.o \
        src/ebtree.o src/eb32tree.o