This adds a bit of overhead, but also adds flexibility to the API,
because arbitrary triggers may be invoked from that virtual method
implementation, not just Cond::signal().

The motivation for this is to make the handlers more dynamic, for the
upcoming buffering class utilizing ProxyInputStream.

Enforcing MADV_DONTFORK is a surprising limitation for this library
which aims to be generic.

This eliminates some overhead, because the compiler doesn't need to
consider these functions throwing.

Switch the remaining users to "postponed_exception".

Will replace "postponed_error".

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

Fixes a problem with the "curl" input plugin: IsEOF() always returns
true because the "open" flag was cleared by
CurlInputStream::RequestDone() when end-of-stream was reached.  This
flag stays false even when seeking to another position has succeeded.

This patch resets the "open" flag to true after seeking successfully.

Fixes crash in the "audiofile" decoder while logging the seek error.

Checking "!open" did not work with the NFS plugin because that plugin
does not close the file automatically, unlike CURL.

This reverts commit 966c4244.

The commit was bad, because the bug was really in
NfsInputStream::DoRead(); see previous commit.

May cause assertion failure.

New base class for other InputStream implementations that run in the
I/O thread.