Use a trivial header file to share mnt_free_tablep and mnt_free_iterp.
It would be nicer put this in mount-util.h, but libmount.h is not in the
default include path, and the build system would have to be adjusted to pass
pkg-config include path in various places, and it's just not worth the trouble.
A separate header file works nicely.
Let's move the shutdown binary into its own subdirectory in
src/shutdown, after all it is relatively isolated from the normal PID 1
sources, being a different binary and all.
Unfortunately it's not possible to move some of the code, since it is
shared with PID 1, that I wished we could move, but I still think it's
worth it.
Yu Watanabe