This refactoring change just moves the cleanup functions around in the
common/utils directory.
libxml2 cleanups are moved to a separate object file, so that we can
still link to libutils even if the main program is not using libxml2
anywhere. Similarly gnulib cleanups.
cleanup.c is renamed to cleanups.c.
A new header file cleanups.h is introduced which will replace
guestfs-internal-frontend-cleanups.h (fully replaced in a later commit).
Just code motion.
This commit makes it clearer what is a utility and what is part of the
library. It also makes it clear that we should rename:
guestfs-internal-frontend.h -> utils.h
guestfs-internal-frontend-cleanups.h -> cleanups.h (?)
but this commit does not make that change.
