Exception safety in class \"thread\"

[nobody]

If an exception is thrown during the copying of the
"threadfunc" object in function "thread_proxy", the new
thread silently ends and the constructor of class
"thread" never returns.

Therefore I would suggest to put a copy of "threadfunc"
in the "thread_param" object (instead of a reference)
and move it to the local variable in function
"thread_proxy" via "threadfunc.swap(…)".

In this way a potential exception would be thrown in
the constructor of class "thread" and could be handled
by the application.

[Roland Schwarz]

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The thread_proxy no longer silently dies on exceptions.