common: New flags for gnupg_spawn_process
* common/exechelp.h (GNUPG_SPAWN_KEEP_STDIN): New. (GNUPG_SPAWN_KEEP_STDOUT): New. (GNUPG_SPAWN_KEEP_STDERR): New. * common/exechelp-posix.c (do_exec): Add arg flags and implement new flags. * common/exechelp-w32.c (gnupg_spawn_process): Implement new flags.