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
