core,w32: Fix popping up of console windows due to gpgconf.

* src/gpgme-w32spawn.c (my_spawn): Create w/o DETACHED_PROCESS.

The spawn helper is actually called with DETACHED_PROCESS and should
not need to do a DETACHED_PROCESS of its own. Interestingly this
patch removes the popups.

See Andre's report

• GnuPG-bug-id: T3515
• Signed-off-by: Werner Koch <wk@gnupg.org>