The log files generated with the previous -Xloggc command contained headers with some information about the JVM. Add similar functionality to unified logging.

The header should be included in rotated outputs as well.

Example of a header from the old log format:
Java HotSpot(TM) 64-Bit Server VM (25.60-b23) for linux-amd64 JRE (1.8.0_60-b27), built on Aug 4 2015 12:19:40 by "java_re" with gcc 4.3.0 20080428 (Red Hat 4.3.0-8)
Memory: 4k page, physical 125650660k(120686628k free), swap 0k(0k free)
CommandLine flags: -XX:+AlwaysPreTouch -XX:ConcGCThreads=8 -XX:InitialHeapSize=53687091200 -XX:InitiatingHeapOccupancyPercent=35 -XX:MaxGCPauseMillis=500 -XX:MaxHeapSize=53687091200 -XX:MaxMetaspaceSize=536870912 -XX:MetaspaceSize=536870912 -XX:ParallelGCThreads=15 -XX:+ParallelRefProcEnabled -XX:+PrintAdaptiveSizePolicy -XX:+PrintGC -XX:+PrintGCDateStamps -XX:+PrintGCDetails -XX:+PrintGCTimeStamps -XX:+UseG1GC