Draft: sanei_thread: Draft fixup for sanei_thread issues.

This is a first draft of an attempt to fix a number of issues in the sanei_thread API.

In particular, some backends are making some invalid assumptions about the underlying type of SANE_Pid. So I have converted SANE_Pid to a struct which highlights where backends (and some parts of sanei_thread) are assuming that SANE_Pid is a pthread_t or at least an arithmetic type.