Add raw RX-50 support. These are 400k single sided disks with 80

tracks and 10 sectors per track. More exotic RX-50 types not
supported, nor is there support for de-interleaving the first two
tracks where the physical sectors are 0 1 2 3 4 5 6 7 8 9, but they
should be interpreted as 0 5 1 6 2 7 3 8 4 9. This is purely to read
the media with dd. The FAT that's on these disks won't work with
msdosfs anyway.

sys/dev/fdc/fdc.c

@@ -155,6 +155,7 @@ static struct fd_type fd_searchlist_12m[] = {
 	{ FDF_5_1230 | FL_AUTO },
 #else
 	{ FDF_5_1200 | FL_AUTO },
+	{ FDF_5_400 | FL_AUTO },
 	{ FDF_5_360 | FL_2STEP | FL_AUTO},
 #endif
 	{ 0 }

sys/sys/fdcio.h

@@ -209,6 +209,7 @@ enum fd_drivetype {
 #define FDF_5_800  10,2,0xFF,0x10,80,0,FDC_300KBPS,2,0x2e,1,0,FL_MFM
 #define FDF_5_720   9,2,0xFF,0x20,80,0,FDC_300KBPS,2,0x50,1,0,FL_MFM
 #define FDF_5_640   8,2,0xFF,0x2A,80,0,FDC_300KBPS,2,0x50,1,0,FL_MFM
+#define FDF_5_400  10,2,0xFF,0x10,80,0,FDC_300KBPS,1,0x2e,1,0,FL_MFM /* RX50 */
 #define FDF_5_360   9,2,0xFF,0x23,40,0,FDC_300KBPS,2,0x50,1,0,FL_MFM
 /* XXX:                      0x2a ? */
 #endif