[cdc_stream] [cdc_rsync] Add --forward-port flag (#45)

Adds a flag to set the SSH forwarding port or port range used for
'cdc_stream start-service' and 'cdc_rsync'.

If a single number is passed, e.g. --forward-port 12345, then this
port is used without checking availability of local and remote ports.
If the port is taken, this results in an error when trying to connect.
Note that this restricts the number of connections that stream can
make to one.

If a range is passed, e.g. --forward-port 45000-46000, the tools
search for available ports locally and remotely in that range. This is
more robust, but a bit slower due to the extra overhead.

Optimizes port_manager_win as it was very slow for a large port range.
It's still not optimal, but the time needed to scan 30k ports is
<< 1 seconds now.

Fixes #12

common/port_range_parser.h

@@ -0,0 +1,33 @@
+/*
+ * Copyright 2022 Google LLC
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#ifndef COMMON_PORT_RANGE_PARSER_H_
+#define COMMON_PORT_RANGE_PARSER_H_
+
+#include <cstdint>
+
+namespace cdc_ft {
+namespace port_range {
+
+// Parses |value| into a port range |first|-|last|.
+// If |value| is a single number a, assigns |first|=|last|=a.
+// If |value| is a range a-b, assigns |first|=a, |last|=b.
+bool Parse(const char* value, uint16_t* first, uint16_t* last);
+
+}  // namespace port_range
+}  // namespace cdc_ft
+
+#endif  // COMMON_PORT_RANGE_PARSER_H_