Reimplement async procedures using pthreads

Reimplement async procedures using pthreads

On Windows, async procedures have always been run in threads, and the
implementation used Windows specific APIs. Rewrite the code to use pthreads.

A new configuration option is introduced so that the threaded implementation
can also be used on POSIX systems. Since this option is intended only as
playground on POSIX, but is mandatory on Windows, the option is not
documented.

One detail is that on POSIX it is necessary to set FD_CLOEXEC on the pipe
handles. On Windows, this is not needed because pipe handles are not
inherited to child processes, and the new calls to set_cloexec() are
effectively no-ops.

Signed-off-by: Johannes Sixt <j6t@kdbg.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>