Issue a warning if legacy logging and Unified Logging are told to use the same file

http://mail.openjdk.java.net/pipermail/hotspot-runtime-dev/2017-December/025557.html

What is worse is that if LogFile and the output of unified logging framework point to the same file, the file may contain partial or corrupted messages from the unified logging framework. For example, consider the following command:

java -Xlog::file=test.log -XX:+UnlockDiagnosticVMOptions -XX:+LogVMOutput -XX:LogFile=test.log

---

The case of conflicting -Xlog::file=test.log and -XX:LogFile=test.log is also a concern, the JVM should at least issue a warning about it.