stream: redo URL parsing, replace m_struct usage with m_config

Move the URL parsing code from m_option.c to stream.c, and simplify it
dramatically. This code originates from times when http code used this,
but now it's just relict from other stream implementations reusing this
code. Remove the unused bits and simplify the rest.

stream_vcd is insane, and the priv struct is different on every
platform, so drop the URL parsing. This means you can't specify a track
anymore, only the device. (Does anyone use stream_vcd? Not like this
couldn't be fixed, but it doesn't seem worth the effort, especially
because it'd require potentially touching platform specific code.)
Remove m_struct

Not needed anymore.
stream: parse URL escapes for file://

So for example "file:///file%20name.mkv" will open "file name.mkv".

I'm not sure whether we want/need this. The old code didn't do it.
Also, it's not really clear whether this is handled correctly. It
seems the corresponding freedesktop.org "standard" allows a (useless)
hostname part, which we should skip in theory. The number of slashes
is not really clear either. We can open relative filenames (by removing
one of the slashes from the example above), which is perhaps an
unneeded feature. How does this even work with Windows paths?

This issues can probably be corrected later.

The URL unescape code is based on code from m_option.c removed with
a recent commit.
m_config: minor simplification

The optstruct parameter in the m_config_parse_option functions was not
needed.
m_option.c: fix typo in message
configure: lower libdvdread minimum required version

This version number was essentially random. When I switched the test
to pkg-config, I took the libdvdread version from my Debian unstable
system as the minimum (as I knew that this version worked).

A user reported that the libdvdread version 4.1.4 appeared to work
fine, so lower the minimum version to the 4.1.x series.
options: don't make options set during playback file local (e.g. --vf)

Refactor file local options handling: instead of making all options
implicitly file local between loading a file and terminating playback,
explicitly make options file local which are required to be file local.

Or in other words, introduce a M_SETOPT_BACKUP flag, which forces file
local-ness when setting an option, and use this for file local command
line options, per-file config files, and per-protocol/extension/vo/ao
profiles.

In particular, this changes the "vf" input command such that video
filters stay permanent even when going to the next file in the playlist.
The underlying reason for this is that the "vf" command uses the option
setting command. This influences the "af" command as well.