Setting the gate of an envelope to a negative value makes a forced
release. What remained unnoticed here was that the forced release shape
is always linear, regardless of the envelope’s shape. This commit
replaces the forced release by a normal release. The fadeTime is set,
so forcing wasn’t necessary.

This fixes supercollider#3775.

testing:
- test how node object replacement and freeing sounds.
- this may change the behaviour of SynthDefs that are separately made
and which do not define a `fadeTime` parameter to specify the release
time. A subsequent commit will try to minimize such impact.

As mentioned in the previous commit, some externally created SynthDefs
may not have a `fadeTime` argument. Their behaviour would change with
the current PR. To minimize impact, we detect if there is a fadeTime
argument in the SynthDef, and fall back to the old behaviour if we find
none.

Here are examples for the two cases:

```
SynthDef(\testFadeOut, { Out.ar(\out.kr, EnvGen.kr(Env.asr,
\gate.kr(1), doneAction:2) * PinkNoise.ar * 0.3) }).add;

Ndef(\x, \testFadeOut).play;
Ndef(\x).objects.first.hasFadeTimeControl == false // true
Ndef(\x).fadeTime = 4;
Ndef(\x, { Silent.ar });

SynthDef(\testFadeOut, { Out.ar(\out.kr, EnvGate.new * PinkNoise.ar *
0.3) }).add;

Ndef(\x, \testFadeOut).play;
Ndef(\x).objects.first.hasFadeTimeControl == true // true
Ndef(\x).fadeTime = 4;
Ndef(\x, { Silent.ar });
```