RFR: 8246592: Simplify checking of boolean file attributes

[Claes Redestad]

Hi,

this patch proposes adding a FileSystem.getBooleanAttribute convenience
method, which simplifies code at use sites. This also enables
improvements when not requesting an attribute that requires extra work,
such as getting the hidden bit on unix derivatives.

Bug:    https://bugs.openjdk.java.net/browse/JDK-8246592
Webrev: http://cr.openjdk.java.net/~redestad/8246592/open.00/

Testing: tier1-2

Micro results on Linux:

Benchmark                   Mode  Cnt  Score   Error  Units
FileOpen.booleanAttributes  avgt   10  1.868 ± 0.002  us/op

Benchmark                   Mode  Cnt  Score   Error  Units
FileOpen.booleanAttributes  avgt   10  0.938 ± 0.008  us/op

Thanks!

/Claes