CH 33 [exec] Signal-safe asynchronous execution

[jensmaurer]

The Committee Draft proposes in clause 33 [exec] components for asynchronous execution of function objects.
The asynchronous execution might be triggered by asynchronous signals.
However, the existing components in clause 33 [exec] don't provide any mechanism to trigger such execution
in a signal-safe way.

This is a serious defect.

This defect needs to be fixed before the standard is published, e.g. by adopting a mechanism as proposed in
P3669.

[inbal2l]

@ogiroux @rarutyun - can you please look at this first?
(see our discussion from: cplusplus/papers#2299 (comment))