Create dsk.h to declare functions defined in dsk.c

Update files that depend on dsk functions to include dsk.h
Declare as static all functions in dsk.c that are not needed externally.
Add dependencies to makefile.

	modified:   makefile-tail
	new file:   ../inc/dsk.h
	modified:   ../src/dir.c
	modified:   ../src/dsk.c
	modified:   ../src/subr.c
	modified:   ../src/ufs.c
	modified:   ../src/vmemsave.c

src/subr.c

@@ -47,6 +47,7 @@ static char *id = "$Id: subr.c,v 1.3 1999/05/31 23:35:42 sybalsky Exp $ Copyrigh
 #include "subrs.h"
 #include "profile.h"
 #include "dbprint.h"
+#include "dsk.h"
 
 extern LispPTR *PENDINGINTERRUPT68k;