jdk.management.agent FileSystemImpl should use standard Java platform File checks

jdk.management.agent contains platform-specific implementations of FileSystemImpl.java and the native code in FileSystemImpl.c to perform some filesystem checking. This mix of Java and native code has very limited usage.

The public methods are isSecuritySupported() and isAccessUserOnly().

These are at least as old as JDK5, with some changes over time. They should be replaced with standard Java API usage, or they should be used to create new requirements, if they have needs not met even by the current file/filesystem APIs.