add Ctrl-C handling to prematurely end solve (convenience feature)

[pokutta]

add the usual Ctrl-C handling to stop the solve but returning feasible solution. Basically we just need to catch it and set a flag in the main loops.

[sebastiendesignolle]

I discussed this with @matbesancon at some point and he mentioned this was quite harder than it looks. But what I'm usually doing is the following: since we do operations on the iterate (or active set) in place, we can easily recover the status of computations if they have a name in the REPL. But I guess there is no smart way of including this practice in the package itself.