Replace suspicious popen_pid assignment with an obviously correct one

Replace suspicious popen_pid assignment with an obviously correct one

popen_pid = vfork() does work correctly, but for a subtle reason
that wrong assignment of 0 happens in the child _first_,
and _then_ correct value overwrites it in the parent.

(And in a hyphothetical system where vfork = fork,
popen_pid wouldn't be shared, so it will also be ok.)

However, it's not necessary to be difficult.
This change makes it so that assignment is done only in parent.

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