Add `Indirect={bool}` to bindings `[Field]` attribute

[spouliot]

For some bindings we need to call Dlfcn.GetIndirect instead of Dlfcn.GetIntPtr. Only the later is supported by the generator so the former (indirect) must be bound manually.

We could add an optional (default to false) Indirect property to FieldAttribute to control which call would be used. That would allow us to reduce (a bit) the manual bindings we maintain and make the xtro results more accurate (i.e. not ignoring stuff we actually support).

[VincentDondain]

Sounds good to me (: 👍