windows: read the PATH env var of the child

The unix and windows process implementations diverge in their behavior
when dealing with subprocesses that are spawned with a relative path.
With unix the *child's* PATH environment variable is read, whereas
with windows the *parent's* environment variable is read.

This commit brings the two implementation in line with respect to
their behavior of reading PATH by having both read the *child's* PATH
environment variable. This involves looking into the user-provided
environment on windows and extracting the PATH variable specifically
so it can be inspected later on.
fix to support android cross compile on mac
Continue using blocking writes for all ttys