V2: Add option to not reset board on connect

I made the mistake of making my original change (#134) on my master branch,
instead of putting it on its own branch, leaving master free to change.
Here is a second version, from its own branch.  It is still untested with
stlink V1, but should build with st-term.

--

'-n' in st-util will cause it to skip the reset step, and thus allow you
to begin debugging at whatever point the code may currently be at.

Adding this feature required changing the stlink_open functions to
accept a reset flag that tells them whether or not to reset after
connecting.  Skipping reset does not seem to have any adverse effects on
stlink usb devices.  Unfortunately, I have to stlink v1 devices to test.