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
> Unfortunately, the trick stumbles over https://issues.dlang.org/show_bug.cgi?id=13666.

Here is a patch: https://github.com/D-Programming-Language/dmd/pull/4099