This restores the behaviour removed in commit
ca831d236a.

When using eSpeak for audio and eSpeak in a server, using the async
speech requests, that code stopped the audio as soon as possible
from a cancel request. This allows servers like brltty to be more
responsive.

This reverts commit 1b5ea47ea4.

This patch is causing lockups when using this with servers like brltty.

This allows process_espeak_command to stop as soon as possible

Using emacspeak server with espeak-ng (still not published) found that audio was only being stoped after a command is processed (i.e. using pcaudiolib).
This fixes this issue.

Moved include/espeak/speak_lib.h to include/espeak-ng/speak_lib.h and
created a symlink from the original location.

This function should just discard the currently playing audio,
rather than close the audio device.  After all,
we will probably want to synthesize something else later.

Removing these calls do not impact on the behaviour of the program
and simplify the wave.h API.

These calls are used to pre-emptively exit when writing audio
data. This is also handled within dispatch_audio. The difference
is that this callback could allow espeak to cancel quicker. In
practice it does not make much difference.

This call was used to update the written sample count used by
wave_get_remaining_time. As that function has been removed, this
code is no longer needed.

This reverts commits 0cc0300328 to
38d15f8f90.

The 0cc03003 commit breaks MBROLA voice support. As such, there
may be other breakages in those commits.