Improved environment variable handling

The environment variables are today copied out from the initial program stack or boot loader data by the scheduler process. When a Loko process starts it asks the scheduler for a copy of the environment and passes it to init-set!.

It would be better to not copy unused parts of the environment into processes other than the scheduler. Variations in the environment are a source of nondeterminism that affects the memory layout of processes. (More environment variables means a GC is triggered slightly earlier, etc). Such variations should be avoided in Loko because they make it harder to track down bugs and they complicate profiling.