Removes need for redundant ParentWithContext implementations. It had led
to it being unsupported on FreeBSD and OpenBSD even though
PpidWithContext was available for them, and different implementations
for getting the parent info used in ParentWithContext and
PpidWithContext on Darwin and Linux.
Add support (for Linux and Windows for now) for reading the
environment variables of a process, with a similar syntax to
os.Environ().
For Windows, this includes some refactoring for clean access to the
RTL_USER_PROCESS_PARAMETERS structure which points to the command
line and the environment variables.
Antoine Toulme