Construct PID trees

[ghost]

https://sourceforge.net/p/strace/mailman/message/35700567/

As it turns out, PTRACE_EVENT_(CLONE|FORK|VFORK) is delivered only
after new task is run, which results in a race between attach-stop
from the new task (which triggers allocation of the new tcb and
creation of the new file) and this aforementioned ptrace event from
the old one, and using child's proc for deriving parent PID looks even
more racy (imagine parent has been killed while we trying to open
/proc/pid/stat and try to go through it). So, this one is not as easy
as it seemed on the first look.

[ashutosh887]

@esyr I would like to work on this Issue