Update files that depend on dir functions to include dir.h
Declare as static all functions in dir.c that are not needed externally.
Add dependencies to makefile-tail.
modified: bin/makefile-tail
new file: inc/dir.h
modified: src/dir.c
modified: src/dsk.c
modified: src/main.c
modified: src/subr.c
modified: src/vmemsave.c
