spawn_next_command: make process group creation optional

Create process group only in wizard - where we originally
needed it.
The "sync" event running functions, OTOH, won't create
a new process group after this. This means that abrt-handle-event,
report-cli and similar CLI tools will not run events in a separate
process group and thus will be killable as a unit via ^C, SIGTERM etc.

Fixes libreport/issues/132.

Signed-off-by: Denys Vlasenko <dvlasenk@redhat.com>