RFR: JDK-8049504,Native crash when opening GPIO pin for input

[Jen Dority]

issue: https://bugs.openjdk.java.net/browse/JDK-8049504
webrev: http://cr.openjdk.java.net/~jld/8049504

Hi,

Need a review for these changes which fix several issues around GPIO 
input handling.

Thanks,
Jen