Minimize use of preexec_fn

Minimize use of preexec_fn

Python suggests avoiding preexec_fn, and in Python 3, changes to the
PATH no longer affect the subprocess invocation, so replace it with an
env setting where we can, and they've also added start_new_session to
provide setsid(), so we'll use that too when possible.

Signed-off-by: Rob Browning <rlb@defaultvalue.org>
Tested-by: Rob Browning <rlb@defaultvalue.org>