stream_dvdnav: implement STREAM_CTRL_GET_NUM_TITLES for dvdnav

This commit provides impelmentation of STREAM_CTRL_GET_NUM_TITLES
for dvdnav stream. Other streams for DVD or Blu-ray are already
provide STREAM_CTRL_GET_NUM_TITLES.
command: add new property 'title'

This commit adds new property 'title' which indicates current
playing title of disc. This property is useful when using a stream
whose title can be changed during playback, e.g., dvdnav.
command: prefix DVD title properties with "dvd-"

They're strictly DVD-only, so it's better to mark them as such. This
also documentes the "title" (now renamed to "dvd-title") property.

This also avoids collision with the --title option. (Technically, there
was no problem. But it might be confusing for users, since we have a
policy of naming properties and options the same if they refer to the
same underlying functionality.)
options: fix off-by-1 error in option help output
DOCS/coding-style: add another requirement for commit messages