[SCIP] avoid presolver unboundedness in conshdlr

[Brannon King]

I have a bilinear constraint for my program: dot(x, y) = 0, where y is
binary and x is unrestricted. As an exercise, I want to convert this
to a ghost constraint -- make it entirely handled by a constraint
handler plugin (without putting any rows into the LP for it). However,
without this constraint, my problem is unbounded. This is detected in
the presolver.

What result do I need to promulgate in the plugin's conspresol method
to inform the presolver that my problem is not actually unbounded?

What is the recommended way to get a list of fixed variables and their
current value in the conspresol?

conslock is triggered without a constraint (in my handler). Should I
just call addVarLocks on all my x and y variables at that time?
conslock seems important even though I don't have any constraints
corresponding to my handler.