frp/pkg/config/legacy

So far, there is no mature Go project that does well in parsing *.ini files.

By comparison, we have selected an open source project: https://github.com/go-ini/ini.

This library helped us solve most of the key-value matching, but there are still some problems, such as not supporting parsing map.

We add our own logic on the basis of this library. In the current situationwhich, we need to complete the entire Unmarshal in two steps:

• Step#1, use go-ini to complete the basic parameter matching;
• Step#2, parse our custom parameters to realize parsing special structure, like map, array.

Some of the keywords in tag(like inline, extends, etc.) may be different from standard libraries such as json and protobuf in Go. For details, please refer to the library documentation: https://ini.unknwon.io/docs/intro.