Moved the FDC FIFO implementation to fifo.c/h, fixes a few length masking bugs in fifo.c, and fixed FDC MSR register RQM bit behavior in DMA mode, which makes 386BSD work, fixes #530.

src/include/86box/fdc.h

@@ -115,14 +115,12 @@ typedef struct fdc_t {
     uint8_t rw_drive;
 
     uint8_t lock;
+
     uint8_t specify[2];
-
     uint8_t res[11];
-
     uint8_t eot[4];
     uint8_t rwc[4];
     uint8_t params[8];
-    uint8_t fifobuf[16];
 
     uint16_t pcn[4];
 
@@ -145,6 +143,8 @@ typedef struct fdc_t {
 
     int drvrate[4];
 
+    void *fifo_p;
+
     sector_id_t read_track_sector;
     sector_id_t format_sector_id;