Update gnulib.

Use the -k whitelist option to get a kernel whitelist.

This is a bug in current febootstrap-supermin-helper.

This option doesn't work yet.  The commit is just for adding the
capability to pass the extra kernel/initrd/appliance parameters
through 'main.c' and 'appliance.c'.

Only -f cpio is permitted by this commit.

We create a 'writer' abstraction and (currently) a single implementation
of this which can write cpio files.  All cpio-related code is moved
out of 'appliance.c' into 'cpio.c'.  'appliance.c' becomes a generic
appliance builder.

This commit should not change the semantics of the program.

We are going to be writing a general appliance, not necessarily
an initrd.  In the ext2 appliance case we will _also_ write an
initrd, so we shouldn't confuse the two things.

This is just code movement.  There is no change to the code.

This will allow us to split up the monolithic C program into
separate files for ease of modification.

This patch is just file rearrangement.