Code maintenance: OS package for file system

- OS custom internal package for file system interaction
- Remove fileManager external dependency
- Closer API to Go's native API on the OS
- Create directories at startup
- Better testability
- Move Unsetenv to os interface

internal/params/shadowsocks.go

@@ -36,7 +36,7 @@ func (r *reader) GetShadowSocksPort() (port uint16, err error) {
 // SHADOWSOCKS_PASSWORD.
 func (r *reader) GetShadowSocksPassword() (password string, err error) {
 	defer func() {
-		unsetErr := r.unsetEnv("SHADOWSOCKS_PASSWORD")
+		unsetErr := r.os.Unsetenv("SHADOWSOCKS_PASSWORD")
 		if err == nil {
 			err = unsetErr
 		}