RFR: 8245724: Move g1 periodic gc logging to G1InitLogger

[stefan.johansson at oracle.com]

Hi,

Please review this change to move more G1 logging to gc+init.

Webrev: http://cr.openjdk.java.net/~sjohanss/8245724/00/
JBS: https://bugs.openjdk.java.net/browse/JDK-8245724

Summary
The initial periodic GC logs are good candidates for being part of the 
G1InitLogger so I moved them there. Since this logging doesn't really 
fit in any of the existing categories I added print_gc_specific() to the 
InitLogger so that the GCs that want to add more stuff have a good place 
to put it.

Testing
Mach5 tier1-2 and local verification.

Thanks,
Stefan