init: move killing of process groups to libprocessgroup

libprocessgroup kills the cgroup associated with a given pid and uid,
but not the POSIX process group associated with it.  This means that
to kill both, two of the same signals must be sent, which may cause
some issues.

This change kills all POSIX process groups whose group leaders are
found within a cgroup.  It only then kills processes in the cgroup
that are not part of the POSIX process groups that have been killed.

Bug: 37853905
Bug: 62418791
Test: Boot, kill zygote, reboot
Change-Id: Id1d96935745899b4c454c36c351ec16a0b1d3827
(cherry picked from commit 70a5ed49)