command: remove some unneeded code

chapter_display_name() always returns something.

The "chapter < -1" check is not needed, because this is done at the
start of the function.
command: make options writeable in idle mode

Until now, options could be accessed as properties via "options/name",
but the access was read-only. Change it so that write access is possible
in --idle mode. (All options have to support setting options at that
time, simply because of the way MPlayer designed per-file options.
During playback, normal properties take care of changing things,
including things backed by options.)

This is work in progress. There are some issues: at least setting the
"vf" and "af" options won't work for strange reasons.
mplayer: remove unused mp_add_subtitles() parameter
mp_core: fix a comment

track.demuxer is never NULL (anymore).
find_subfiles: don't try to open URLs as directories

Did things like opendir("http://..."). Makes no sense, and just
generates noise.
find_subfiles: use stat() instead of opening the file to check existence

Use mp_path_exists() to check for existence of a file (which in turn
uses stat()), instead of opening and closing it. The difference is that
if we don't have sufficient permissions to read the subtitle files, we
will loudly complain. Personally, I prefer this behavior.
find_subfiles: some cleanups

Remove a crap assert() (what... either it can't happen, or it should
error or at least abort() if it can't be handled).

Remove some dead definitions.
find_subfiles: don't auto-load .sub file if .idx file exists

External vobsubs usually come as .idx/.sub pairs. Loading the .idx file
implicitly loads the .sub file, whereas loading the .sub file will kind
of work, but miss important information such as subtitle resolution. Or
in other words, if the .idx file exists, adding the .sub file as track
is useless and confusing.

Explicitly remove .sub file from the auto-load suntitle list in these
cases. Standalone .sub files are still loaded.

We also drop that weird logic that excluded .utf8 files from being
loaded if -subcp was in use. I hope the associated use case didn't make
much sense to begin with. If not, we could still implement it properly,
instead of this weird hack.
mplayer: don't auto-load explicitly loaded subtitle files

Even if a subtitle was explicitly loaded with -sub, it was still auto-
loaded (if auto-loading applied to that file). Fix this by explicitly
checking whether a file is already loaded.

The check is maximal naive and just compares the filenames as strings.
The change in find_subfiles.c is so that "-sub something.ass" happens to
work (auto-loading prepended a "./" to it, so the naive filename
comparison check didn't work).