[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

cdc_stream/base_command.h

@@ -48,6 +48,13 @@ class BaseCommand {
   std::function<void(const std::string&)> JedecParser(const char* flag_name,
                                                       uint64_t* bytes);
 
+  // Parser for single ports "123" or port ranges "123-234". Usage:
+  //   lyra::opt(PortRangeParser("port-flag", &first, &last), "port"))
+  // Automatically reports a parse failure on error.
+  std::function<void(const std::string&)> PortRangeParser(const char* flag_name,
+                                                          uint16_t* first,
+                                                          uint16_t* last);
+
   // Validator that should be used for all positional arguments. Lyra interprets
   // -u, --unknown_flag as positional argument. This validator makes sure that
   // a positional argument starting with - is reported as an error. Otherwise,
@@ -82,9 +89,9 @@ class BaseCommand {
   // Extraneous positional args. Gets reported as error if present.
   std::string extra_positional_arg_;
 
-  // Errors from parsing JEDEC sizes.
+  // Errors from custom flag parsers, e.g. JEDEC sizes or port ranges.
   // Works around Lyra not accepting errors from parsers.
-  std::string jedec_parse_error_;
+  std::string parse_error_;
 };
 
 }  // namespace cdc_ft