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)

mac/file.h

@@ -59,9 +59,9 @@ std::vector<fs::path> pathnames_with_extension(
 std::vector<std::string> find_files_with_extension(
     const fs::path& root, const std::string& ext, bool case_insensitive=false);
 std::string combine_files(
-    std::vector<std::string> filenames,
-    std::string prolog="", std::string epilog="",
-    std::function <std::string(std::string)> processor = nullptr);
+    const std::vector<std::string>& filenames,
+    const std::string& prolog="", const std::string& epilog="",
+    const std::function <std::string(std::string)>& processor = nullptr);
 
 bool files_differ(const fs::path& p1,
                   const fs::path& p2,