RFR JDK-8066577: Cleanup and make better use of the stream API in the jrtfs code

[Tagir Valeev]

Hello!

> (2) to use Collections.singleton() as Claes suggested. It appears the
"singleton set" is
> better choice (light) in this case.

Btw why not new-and-cool Set.of() ?
http://download.java.net/java/jdk9/docs/api/java/util/Set.html#of-E-

With best regards,
Tagir Valeev.