ao_dsound: reduce default buffer size

Reduce from 1000ms to 100ms. Since there is an audio thread updating AOs
quickly enough now, requesting such a large buffer size makes no sense
anymore.
manpage: add sections and order by usage

Split the options into the following sections:

 * Playback Control
 * Program Behaviour
 * Video
 * Audio
 * Subtitles
 * Window
 * Disc Devices
 * Equalizer
 * Demuxer
 * Input
 * OSD
 * Screenshot
 * Software Scaler
 * Terminal
 * TV
 * Cache
 * Network
 * DVB
 * PVR
 * Miscellaneous

Most options are sorted by usefullness and how often they're used or how
important they are.
This makes finding the right options easier and adds some sort of structure.
manpage: update key bindings
input.conf: switch chapter seek next/previous keys

"UP" seeks forward, so "PGUP" should skip to the next chapter.

Fixes #998.
etc: add mplayer-input.conf
command: run OSD display code even if a property is unavailable

Trying to jump chapters in a gile that has no chapters does nothing,
not even show a warning. This is confusing. The reason is that the
"add chapter" command will just bail out completely if the property
is unavailable.

This was because it exited when it couldn't get the property type.
Instead of exiting, just don't enter the code that needs the type.

(I'm not sure when this behavior changed. I consider it a regression.
It was probably caused by changes to the chapter code, which perhaps
started returning UNAVAILABLE instead of OK if there are no chapters.)
etc/example.conf: update cache options

Some of them changed semantics or got renamed.

Note that the replacements in the example.conf are not necessarily the
equivalents of the replaced options.
video: remove "hard" framedrop mode

Completely useless, and could accidentally be enabled by cycling
framedrop modes. Just get rid of it.

But still allow triggering the old code with --vd-lavc-framedrop, in
case someone asks for it. If nobody does, this new option will be
removed eventually.
input.conf: map shift+pgup/dwn to the old seek bindings
vo_sdl: fix redrawing issue

vo_sdl.c has broken event handling and just polls. The polling time was
quite low, so the playloop OSD redrawing heuristic inhibited redraws,
which made the window appear frozen when paused.
player: don't delay OSD redraw when paused

We want this heuristic to trigger during normal playback only.