Another step towards supporting multiple readers.

There is just one thread.

The thread will always attempt to read more data since commit
2cf6b776, so we don't need to tell it
to continue.

The "seek_error" attribute will be eliminated soon.

More preparations to support multiple readers.

If a read error occurs, it is very unlikely that the InputStream will
ever recover.  Removing the code removes some code complexity which
just isn't worth it.  And it allows supporting multiple readers for
one buffer.

After seeking, a new read should be done.

Prepare to reuse it in another class.

ALSA is just one out of many output plugins, and detailed plugin
documentation should only live in the user manual, without having
duplicates in the (brief) manpage.

Also move "mixer_type" to the "optional audio output parameters"
section; it is a generic option, not specific to ALSA.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/552

The HugeArray constructor can throw std::bad_alloc, and so can
SparseBuffer's constructor.