Switch the remaining users to "postponed_exception".

Got lost in commit 6ed77f2a

Will replace "postponed_error".

Use the snd_pcm_t only in the IOThread, and reuse code that is
well-known to work.

Necessary to preserve type information.  The try/catch sequence didn't
work previously.

Same fix as in commit 1c904000

Obsolete class CloseSongEnumerator, which was a kludge.

default_value defaults to nullptr.

Convenient method that behave differently than Read, and that will be used by
tag scanners.

This method will return in case of error, if the whole data is read or is EOF
is reached.