Make struct CapIOManager be fully opaque

Provide an opaque (forward) definition in Capability.h (since the cap
contains a *CapIOManager) and then only provide a full definition in
a new file IOManagerInternals.h. This new file is only supposed to be
included by the IOManager implementation, not by its users. So that
means IOManager.c and individual I/O manager implementations.

The posix/Signals.c still needs direct access, but that should be
eliminated. Anything that needs direct access either needs to be clearly
part of an I/O manager (e.g. the sleect() one) or go via a proper API.