SI-7410 REPL uses improved tools.jar locator

The logic in partest for snooping around for tools.jar
is moved to PathResolver, and ILoop uses it from there.

If JAVA_HOME is toolless, check out java.home.

The use case was that Ubuntu installs with `java` at
version 6 and `javac` at version 7; it's easy to wind
up with JAVA_HOME pointing at the version 6 JRE, as
I discovered.  It's confusing when that happens.

In future, partest might run under 7 and fork tests
under 6, but those permutations are downstream.
PathResolver uses platform EOL to mkStrings
Merge pull request #2584 from som-snytt/issue/repl-tools-jar

SI-7410 REPL uses improved tools.jar locator