Log Message:
htmldoc: update to 1.9.16.

# Changes in HTMLDOC v1.9.16

- Added support for `$DATE(format)` and `$TIME(format)` header/footer strings
  (Issue #472)
- Fixed a potential image overflow bug with JPEG and PNG images (Issue #471)
- Fixed potential heap overflow bugs with pages (Issue #477, Issue #478,
  Issue #480, Issue #482, Issue #483)
- Fixed potential use-after-free in blocks (Issue #484)
- Updated the GNU TLS HTTPS support code to use a faster connection shutdown
  mode (Issue #487)
- Fixed some minor Coverity warnings.
- Updated the GUI interface for current display fonts.