m_config: store file local backup options differently

File local options need to backup the global option value while a file
is played. Instead of keeping a pointer in m_config_option for the
backup value, put it into a separate list. This reduces per-option
overhead for a rarely used obscure feature. (This implementation would
have been a bit dumb in pre-mpv m_config, because there local options
were the default, and _all_ options were backed up when starting
playback of a file. This is not the case anymore, and normally only
a very small number of options are backed up by default.)

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