parallel: add support for optional reset/enable/select signal

parallel: add support for optional reset/enable/select signal

Add an optional 'reset' signal of user configurable polarity. When the
signal is asserted, the data lines are not interpreted. Assertion will
flush previously accumulated data bits and words. Deassertion can help
synchronize to input streams when the capture started in the middle of
a word. Despite the "reset" name this signal can also be thought of as
"enable" or "select", and increases the versatility and usefulness of
the parallel decoder beyond strictly parallel memory busses.

Construct the list of .wait() conditions and track the positions of
individual terms in that list. This is necessary because "always false"
conditions are not available, thus pin/channel indices and .matched[]
indices don't correspond for sparse input signal assignments.

Accept when previously gathered information became void again, and
re-use existing initialization code for reset related activity.