[Maintenance] Use a dedicated enum for snd_load()'s function parameter

It's not a kaja_func_t if it's shifted left by 8 bits. Why is it
shifted left by 8 bits to begin with, though? Why not just pass a
kaja_func_t, and assign it to AH? Arrrrgh.

Part of P0139, funded by [Anonymous].

libs/kaja/kaja.h

@@ -28,8 +28,6 @@ typedef enum {
 	// (DL) buffers, in KiB. These can be customized on the PMD command line
 	// with the /M, /V and /E options respectively.
 	PMD_GET_BUFFER_SIZES = 0x22,
-
-	_kaja_func_t_FORCE_UINT16 = 0xFFFF
 } kaja_func_t;
 
 #define PMD 0x60

th02/snd/load.cpp

@@ -8,7 +8,7 @@ extern "C" {
 
 extern char snd_load_fn[SND_FN_LEN];
 
-void snd_load(const char fn[SND_FN_LEN], kaja_func_t func)
+void snd_load(const char fn[SND_FN_LEN], snd_load_func_t func)
 {
 	int i;
 	__asm { push ds; }

th02/snd/snd.h

@@ -53,13 +53,7 @@ void snd_delay_until_volume(uint8_t volume);
 	void snd_delay_until_measure(int measure);
 #endif
 
-// Shorter symbols for the [func] parameter of snd_load()
-#define SND_LOAD_SONG (kaja_func_t)(KAJA_GET_SONG_ADDRESS << 8)
-#define SND_LOAD_SE (kaja_func_t)(PMD_GET_SE_ADDRESS << 8)
-
-#define SND_FN_LEN 13
-
-#if defined(PMD) && (GAME <= 3) /* requires kaja.h */
+#if defined(PMD) /* requires kaja.h */
 	#if defined(__cplusplus) && (GAME <= 4)
 		static inline uint16_t snd_load_size() {
 			// ZUN bug: Should rather retrieve the maximum data size for song
@@ -73,11 +67,21 @@ void snd_delay_until_volume(uint8_t volume);
 		}
 	#endif
 
-	// Loads a song in .M format ([func] = SND_LOAD_SONG) or a sound effect
-	// bank in EFC format ([func] = SND_LOAD_SE) into the respective work
-	// buffer of the sound driver. If MIDI is used, 'md' is appended to the
-	// file name.
-	void snd_load(const char fn[SND_FN_LEN], kaja_func_t func);
+	typedef enum {
+		SND_LOAD_SONG = (KAJA_GET_SONG_ADDRESS << 8),
+		SND_LOAD_SE = (PMD_GET_SE_ADDRESS << 8),
+	} snd_load_func_t;
+
+	#define SND_FN_LEN 13
+
+	#if (GAME <= 3)
+		// Loads a song in .M format ([func] == SND_LOAD_SONG) or a sound
+		// effect bank in EFC format ([func] == SND_LOAD_SE) into the
+		// respective work buffer of the sound driver. If MIDI is used, 'md'
+		// is appended to the file name.
+		// [fn] still needs to be null-terminated, despite its fixed length.
+		void snd_load(const char fn[SND_FN_LEN], snd_load_func_t func);
+	#endif
 #endif
 
 void snd_se_reset(void);

th04/snd/snd.h

@@ -40,7 +40,7 @@ static inline bool16 snd_se_active() {
 int pascal snd_determine_modes(int req_bgm_mode, int req_se_mode);
 
 #if defined(PMD) /* requires kaja.h */
-// Loads a song ([func] = SND_LOAD_SONG) or a sound effect bank ([func] =
+// Loads a song ([func] == SND_LOAD_SONG) or a sound effect bank ([func] ==
 // SND_LOAD_SE) into the respective work buffer of the sound driver. [fn] must
 // not have any extension. Depending on [snd_bgm_mode], [snd_se_mode], and
 // game, the following file is loaded:
@@ -60,7 +60,7 @@ int pascal snd_determine_modes(int req_bgm_mode, int req_se_mode);
 //
 // Note that the TH05 version will infinitely loop if neither the file for the
 // current [snd_bgm_mode] nor "[fn].m" exist.
-void pascal snd_load(const char fn[SND_FN_LEN], int16_t func);
+void pascal snd_load(const char fn[SND_FN_LEN], snd_load_func_t func);
 
 #if defined(__cplusplus) && (GAME == 5)
 	// Refer to TH02's implementation for an explanation of how wrong this is.

th05/snd/load.cpp

@@ -12,7 +12,7 @@ extern "C" {
 extern char snd_load_fn[SND_FN_LEN];
 extern const char SND_LOAD_EXT[4][4];
 
-void pascal snd_load(const char fn[SND_FN_LEN], int16_t func)
+void pascal snd_load(const char fn[SND_FN_LEN], snd_load_func_t func)
 {
 	#define _DI	reinterpret_cast<char near *>(_DI)
 	#define func_local	_BP