player: some more input refactoring

Continues commit 348dfd93. Replace other places where input was manually
fetched with common code.

demux_was_interrupted() was a weird function; I'm not entirely sure
about its original purpose, but now we can just replace it with simpler
code as well. One difference is that we always look at the command
queue, rather than just when cache initialization failed. Also, instead
of discarding all but quit/playlist commands (aka abort command), run
all commands. This could possibly lead to unwanted side-effects, like
just ignoring commands that have no effect (consider pressing 'f' for
fullscreen right on start: since the window is not created yet, it would
get discarded). But playlist navigation still works as intended, and
some if not all these problems already existed before that in some
forms, so it should be ok.
stream: change cache return values

Basically a cosmetic change, because currently the player just continues
even if the cache fails initializing.