Make the GC configurable at runtime via env vars

Make the GC configurable at runtime via env vars

The GC offers a couple of options to debug memory problems, but they are
selectable only at compile-time. Being the GC part of the compiler
runtime, is not very common for the user to recompile the GC when it has
a memory problem, so making this option available always is very
desirable.

This patch allows configuring the GC via environment variables. 4 options
are available: sentinel, mem_stomp, verbose and log file. Only the first
2 are implemented right now.

For example, to check a program using memory stomping and a sentinel, you
can run it like this (using sh):

	$ D_GC_OPTS=mem_stop=1:sentinel

As you can see, the value is optional for boolean options.