Fix html::page declaration/definition divergence from the const sweep

The four defaulted parameters in html_util.h stayed by-value while the
definition became const&, leaving the called overload undefined.
Linux's -shared linking hid it (lazy dlopen resolution); it broke the
book-structure HTML path at runtime. Both sides now agree (const&,
defaults kept); combine_files converted consistently as well.

(from dev a910e11431d2)

README.md

@@ -41,7 +41,7 @@ are regenerated on each release — patches cannot be merged directly.
 Report problems (or send patches) to the author; accepted changes are
 applied to the development tree and appear in a following snapshot.
 
-This snapshot was assembled from development commit `64b1abf23e56`.
+This snapshot was assembled from development commit `a910e11431d2`.
 
 ## License