Revise the IP solver interface code.

[GunnarFarneback]

• Repair the CBC backend. (It tried to pass "solver_options" literally to CBC.)
• Don't pop! from solver_options; it mutates the caller's variable. If the caller tries to use it for a second call without recreating it, it won't work the same.
• Set the algorithm controlled solver attributes directly instead of having them as a default for solver_options, which could easily get lost when that is customized. It's still possible to override them with explicit solver_options.
• Get rid of the name conversions and send all solver_options as raw solver attributes.
• Test the JuMP backend.
• Document the JuMP backend and solver_options.

[jd-foster]

Looks good - thanks for this follow-up. This is exactly how I would have tried to do it, but you've done it better instead!