Launch Kleo/GPA in the background on start
* src/engine-assuan.c (spawn_background): New. (op_assuan_init): Create background thread to launch kleo.
This does not slowdown the startup as it happens in a different
thread but is intended to speed up the opening of the key
resolution dialog when kleopatra is not running and help to
avoid startup problems mentioned in T3401
- GnuPG-Bug-Id: T3401