GC tweaks

In a minor collection, old-generation pages can be ignored for
the fixup pass. Also, the "moved" bit implies the "marked" bit.

Also, move the mark bits to low and the hash bits to high,
which leads to slightly better generated code for the GC.
fix the name of `environment-variables-ref'

Also, fix use of unsetenv() to work with platforms where it returns
void.
code tweaks