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
I guess these drt_* variables no longer belong to gc.config. They should be moved to some more general place. We might add them elsewhere (rt/dmain2.d) or we add a new module rt.config. Any suggestions?
core package doesn't work because we would add new template instances when accessing the variables.