OS.h: more type elaboration (#21)

* 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>

src/d/d_save/d_save.cpp

@@ -1,6 +1,7 @@
 #include "d/d_save/d_save/d_save.h"
 
 #include "d/d_com/d_com_inf_game/d_com_inf_game.h"
+#include "os/OS.h"
 
 u8 dSv_item_rename(u8 item_id) {
     switch (item_id) {
@@ -854,11 +855,11 @@ asm void dSv_player_info_c::init(void) {
 #endif
 
 void dSv_player_config_c::init(void) {
-    u32 os_sound_mode;
+    OSSoundMode os_sound_mode;
 
     this->unk0 = 1;
     os_sound_mode = OSGetSoundMode();
-    if (os_sound_mode == 0) {
+    if (os_sound_mode == SOUND_MODE_MONO) {
         this->sound_mode = 0;
         Z2AudioMgr_NS_setOutputMode(lbl_80451368, 0);
     } else {