Cleanup of gc.config

followup of #817:
- updated comments
- allow disabling command line processing through -version=noinitGCFromCommandline
- change environment var to DRT_GCOPT
- change default of maxPoolSize to 64
Now that you mention shared libraries: the command line interface doesn't work there at all, because it initializes before the application is able to pass any arguments via rt_args().

I think one important use case is to change the defaults of the GC to match what an application is doing. I currently don't see how this can be done other than linking in a different gc.config. Weak linkage might work, too, but I'm unsure about the OMF toolchain.