Introduce UDP proxy wrapper script

This commit introduces the script `programs/test/udp_proxy_wrapper.sh` which can
be used to wrap the SSL server binary `programs/ssl/ssl_server2` by the UDP
proxy application `programs/test/udp_proxy` while maintaining the same
interface from the command line.

Specifically, given UDP proxy arguments ARGS_UDP and SSL server arguments
ARGS_SSL, the command line

> ./udp_proxy_wrapper.sh ARGS_UDP -- ARGS_SSL

behaves like

> ./ssl_server2 ARGS_SSL

wrapped by

> ./udp_proxy ARGS_UDP

The motivation and benefit of this is that scripts like `ssl-opt.sh` can be used
with the server command line `P_SRV` modified to `./udp_proxy_wrapper.sh
ARGS_UDP -- DEFAULT_ARGS_SSL` which will result in all tests being executed for
an SSL server behind a UDP proxy.

programs/test/udp_proxy_wrapper.sh

@@ -0,0 +1,103 @@
+#!/bin/sh
+
+set -u
+
+MBEDTLS_BASE="$(pwd)/$(dirname $0)/../../"
+TPXY_BIN="$MBEDTLS_BASE/test/udp_proxy"
+SRV_BIN="$MBEDTLS_BASE/programs/ssl/ssl_server2"
+
+: ${VERBOSE:=0}
+VERBOSE=1
+
+PARAM_SEP="^(.*)--(.*)$"
+PROXY_PARAMS=$(echo $@ | sed -n -r "s/$PARAM_SEP/\1/p")
+SERVER_PARAMS=$(echo $@  | sed -n -r "s/$PARAM_SEP/\2/p")
+
+stop_proxy() {
+    test -n "${TPXY_PID:-}" &&
+        (
+            echo "\n  * Killing proxy (pid $TPXY_PID) ..."
+            kill $TPXY_PID
+        )
+}
+
+stop_server() {
+    test -n "${SRV_PID:-}" &&
+        (
+            echo "\n  * Killing server (pid $SRV_PID) ..."
+            kill $SRV_PID >/dev/null 2>/dev/null
+        )
+}
+
+cleanup() {
+    stop_server
+    stop_proxy
+    return 1
+}
+
+trap cleanup INT TERM HUP
+
+DTLS_ENABLED=$(echo "$SERVER_PARAMS" | grep -v "::1" | grep "dtls=1")
+if [ -z "$DTLS_ENABLED" ]; then
+    echo "  * Couldn't find DTLS enabling, or IPv6 is in use - immediate fallback to server application..."
+    if [ $VERBOSE -gt 0 ]; then
+        echo "[ $SRV_BIN $SERVER_PARAMS ]"
+    fi
+    $SRV_BIN $SERVER_PARAMS >&1 2>&1 &
+    SRV_PID=$!
+    wait $SRV_PID
+    exit 0
+fi
+
+SERVER_PORT_ORIG=$(echo "$SERVER_PARAMS" | sed -n -r "s/^.*server_port=([0-9]+).*$/\1/p")
+if [ -z "$SERVER_PORT_ORIG" ]; then
+    echo "  * No server port specified - exit"
+    exit 1
+fi
+
+SERVER_ADDR_ORIG=$(echo "$SERVER_PARAMS" | sed -n -r "s/^.*server_addr=([a-zA-Z0-9\.]+).*$/\1/p")
+if [ -z "$SERVER_ADDR_ORIG" ]; then
+    echo "  * No server address specified - exit"
+    exit 1
+fi
+
+echo "  * Server address:    $SERVER_ADDR_ORIG"
+echo "  * Server port:       $SERVER_PORT_ORIG"
+
+SERVER_PORT=$(( $SERVER_PORT_ORIG + 1 ))
+echo "  * Intermediate port: $SERVER_PORT"
+
+TPXY_CMD=\
+"$TPXY_BIN $PROXY_PARAMS "\
+"listen_port=$SERVER_PORT_ORIG "\
+"server_port=$SERVER_PORT "\
+"server_addr=$SERVER_ADDR_ORIG "\
+"listen_addr=$SERVER_ADDR_ORIG"
+
+echo "  * Start proxy in background ..."
+if [ $VERBOSE -gt 0 ]; then
+    echo "[ $TPXY_CMD ]"
+fi
+
+$TPXY_CMD >/dev/null 2>&1 &
+TPXY_PID=$!
+
+if [ $VERBOSE -gt 0 ]; then
+    echo "  * Proxy ID:          $TPXY_PID"
+fi
+
+SERVER_PARAMS_NEW=$(echo $SERVER_PARAMS | sed -n -r "s/^(.*server_port=)[0-9]+(.*)$/\1$SERVER_PORT\2/p")
+SRV_CMD="$SRV_BIN $SERVER_PARAMS_NEW"
+
+echo "  * Starting server ..."
+if [ $VERBOSE -gt 0 ]; then
+    echo "[ $SRV_CMD ]"
+fi
+
+$SRV_CMD >&2 &
+SRV_PID=$!
+
+wait $SRV_PID
+
+stop_proxy
+return 0