Add systemd socket activation capability

Add support for a systemd.socket activation. Socket it passed from
systemd via file descriptor (see systemd.socket doc)
Also, added a few features for convenience:
  * --conn/conv-timeout options to control conversation and connection
    timeouts
  * --shutdown option to shutdown service after all connections are
    timeouted. This is for convent usage with systemd.socket
  * test script which allows to smoke test udp speeder and its
    socket activation capability
  * example systemd unit files

makefile

@@ -26,7 +26,7 @@ export STAGING_DIR=/tmp/    #just for supress warning of staging_dir not define
 # targets for nativei (non-cross) compile 
 all:git_version
 	rm -f ${NAME}
-	${cc_local}   -o ${NAME}          -I. ${SOURCES} ${FLAGS} -lrt -ggdb -static -O2
+	${cc_local} -DSYSTEMD_SOCKET_ACTIVATION -o ${NAME} -I. ${SOURCES} ${FLAGS} -lrt -lsystemd -ggdb -O2
 
 freebsd:git_version
 	rm -f ${NAME}