Refactor subprocess creation

Refactor subprocess creation

Instead of having a gazillion different little functions that wrap
os.spawnvp and popen2.Popen3, make a single wrapper that's flexible
enough to cater to everyone. Apart from making the code cleaner and
easier to read, this has the added benefit of making it possible to
add little things such as extra safety checks and logging.

Ideally, I'd have liked this wrapper to wrap subprocess.Popen, but
that's only available in Python 2.4 and up, so we'll have to make do
with os.spawnvp and popen2.Popen3 for now, even though they suck.

Signed-off-by: Karl Hasselström <kha@treskal.com>