LINUX: Fix remote pid re-attach bug

When pid from file flag was used and remote pid
has been updated (new pid read from file), static
boolean flag will never change preventing reapChild
from attaching to new pid.

Slightly modify the logic to attach again to long-lived
process when new pid detected.

Signed-off-by: Anestis Bechtsoudis <anestis@census-labs.com>