RFR 8080087: Nashorn $ENV.PWD is originally undefined

[A. Sundararajan]

Please approve.

Bug: https://bugs.openjdk.java.net/browse/JDK-8080087
jdk9 review thread: 
http://mail.openjdk.java.net/pipermail/nashorn-dev/2015-June/004677.html
jdk8u-dev webrev: http://cr.openjdk.java.net/~sundar/8080087/8u60/webrev.00/

Backported 'as is' except for modular source layout difference.

Thanks,
-Sundar