include/vlc_input_item: correct misleading documentation about VLC_INPUT_OPTION_UNIQUE
src/input: add a new input option flag to replace values of existing options

This flag can be used with input_item_AddOption to replace the value
of an input option, if this option already exists. Otherwise, the
new option is added as usual.

refs #11471
macosx: fix resume playback feature

close #11471