The WireGuard implementation will have two options that should not have a fixed default value, because they represent two keys (one is preshared, the other is private).
Instead of handling these two options differently in ProtoNewContainer(), this commit adds a new function to PROTO_IMPL: ProtoOptionString().
ProtoOptionString() takes the option's name as argument and returns a heap-allocated string that will be used as value. The function returns NULL when the option doesn't need a randomized value.
PROTO_OPTION is a structure that describes an option (who would've guessed?).
It's designed in a way that allows it to occupy as low memory as possible, while providing great flexibility.
The idea is similar to the one implemented in LIST for trivial types, with the difference that PROTO_OPTION doesn't require casting due to the use of union.
