RFR 8023173: FileDescriptor should respect append flag

[Ivan Gerasimov]

Hello everyone!

I've changed the fix in order to address the concerns Alan had mentioned.

Now, both the Unix and Windows implementations of FileDescriptor class 
have the append flag.
First, it allows such querying the file descriptor in 
FileChannelImpl.position() that does not involve JNI calls.
Second, this flag is passed to the write functions as an argument, so 
there's no need to retrieve it from the native code.

The fix was built on all available platforms.
All the tests from io, nio pass.

BUGURL: https://bugs.openjdk.java.net/browse/JDK-8023173
WEBREV: http://cr.openjdk.java.net/~igerasim/8023173/3/webrev/

Would you please help review this?

Sincerely yours,
Ivan