Root config: Add env config (#6400)

https://github.com/XTLS/Xray-core/pull/6400#issuecomment-4880198100
https://github.com/XTLS/Xray-core/pull/6400#issuecomment-4916892425
https://github.com/XTLS/Xray-core/pull/6400#issuecomment-4929136670

common/buf/readv_reader_stub.go

@@ -10,7 +10,9 @@ import (
 	"github.com/xtls/xray-core/features/stats"
 )
 
-const useReadv = false
+func useReadV() bool {
+	return false
+}
 
 func NewReadVReader(reader io.Reader, rawConn syscall.RawConn, counter stats.Counter) Reader {
 	panic("not implemented")