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
> How about making rt_configOption take a delegate which gets called for each match in order of precedence.

I implemented this, but I'm not sure it is actually an improvement. It makes the code quite a bit more complex. (And showed that the compiler is pretty dumb when detecting unnecessary closures aswell as @nogc not reporting them).