Support MHNG_MAILDIR

This allows users to set an enviornment variable that controls where
their MH configuration should come from.  It's also going to end up
being useful for automated testing, so I can mock up MH directories.
Support the MHNG_NO_DAEMON enviornment variable

This is pretty much only going to be used for testing.  The idea is
that when I run tests I don't want to start forking off processes all
over the place that won't do anything.

Instead this just allows the MHng daemon to be completetly disabled.
Any requests return instantly with a dummy response.
Add a test case!

This is a regression test for

  6f7b92f Work around some Krste's mail client bugs