Allow link-time and run-time GC configuration

followup of #817:
- updated comments
- allow disabling command line processing through variable override at link time 
- allow enabling environment processing through variable override at link time 
- allow adding configuration option at link time 
- change environment var to DRT_GCOPT
- change default of maxPoolSize to 64
Well it's not really complicated and you can declare extern D functions in one module while defining them in a different module, all completely type safe.
As those are runtime option they have no place in core, maybe in core.internal.