loader: add cwd and env options

Add cwd (current directory) and env (environment variable) option to the
loader. Can be useful for certain applications that expect to be in a
certain directory, or certain environment variables to exist.

Example usage:

   run.py -e "--cwd=/tmp /usr/bin/something.so"

Signed-off-by: Nadav Har'El <nyh@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>