common,w32: Breakaway detached childs when in job

* common/exechelp-w32.c (gnupg_spawn_process_detached): Add
CREATE_BREAKAWAY_FROM_JOB creation flag if required.

When the gpg process is assigned to a W32 "Job" the
child processes are killed once the Job is finished.
As we want our detached processes to linger e.g.
gpg-agent the breakaway flag is required in
that case.

Thanks to Jan Echternach for reporting this and providing
a patch.

• GnuPG-Bug-Id: T4333
• Signed-off-by: Andre Heinecke <aheinecke@gnupg.org>