* scm/layout-page-dump.scm (scm): export utility function names,

* scm/layout-page-dump.scm (scm): export utility function names,
to allow user writing cutsom dumping functions.

* scm/layout-page-layout.scm: export utility function names to
enable custom page breaking function writing. Tabify.
(line-next-space): use ?-suffix only for predicates
(page-maximum-space-to-fill): new function. Return the space
between first and bottom system of a page, to give to
space-systems.
(space-systems): use a space-to-fill argument (for instance as
computed by page-maximum-space-to-fill) instead of computing
internaly this space with the page height. That way, the caller
can adjust the space to use.
(make-page-from-systems, walk-paths): compute space to fill before
calling space-systems.