* move OS function decls to their own header, os.h
* os.h: OSReport (+related), OSMutex, OSThread; fixup uses
* os.h: OS{Mutex,Thread}{Link,Queue}, OSCond
* os.h: OSxx typedefs, OSMessageQueue
* os.h: fix time function decls
* os.h: retype misc sched fns, create u32 OSSoundMode enum (and fixup use)
* os.h: more retyping
* merge os.h into os/OS.h
@Julgodis did this in parallel to me and added some functions beyond
what was in functions.h, so I merged our versions together
* os.h: retype the rest of the (currently decl'd) thread functions
* d_save: move OS import
Co-authored-by: Pheenoh <pheenoh@gmail.com>