Validate port options

Check `port` options for invalid values (ports are parsed as uint16, so
in practice values >65535 are invalid; port 0 is undefined and therefore
considered invalid too). This allows for an early rejection of faulty
values and an supplying an informative message to the user.

Splits tests in `feature_proxy.py` to cover both invalid `hostname`
and `port` values.

Adds a release-note as previously valid `-port` and `-rpcport` values
can now result in errors.

src/util/error.cpp

@@ -49,6 +49,11 @@ bilingual_str ResolveErrMsg(const std::string& optname, const std::string& strBi
     return strprintf(_("Cannot resolve -%s address: '%s'"), optname, strBind);
 }
 
+bilingual_str InvalidPortErrMsg(const std::string& optname, const std::string& invalid_value)
+{
+    return strprintf(_("Invalid port specified in %s: '%s'"), optname, invalid_value);
+}
+
 bilingual_str AmountHighWarn(const std::string& optname)
 {
     return strprintf(_("%s is set very high!"), optname);

src/util/error.h

@@ -39,6 +39,8 @@ bilingual_str TransactionErrorString(const TransactionError error);
 
 bilingual_str ResolveErrMsg(const std::string& optname, const std::string& strBind);
 
+bilingual_str InvalidPortErrMsg(const std::string& optname, const std::string& strPort);
+
 bilingual_str AmountHighWarn(const std::string& optname);
 
 bilingual_str AmountErrMsg(const std::string& optname, const std::string& strValue);