use process groups for processes started by init

Put every service into a process group, kill the process group
and all child processes created within the group when killing the
service. Removed libutil dependency in libprocessgroup.

Bug: 25355957
Change-Id: Ieed60ec41579f638ab9b1e66a7e6330ed578ab05
Signed-off-by: Collin Mulliner <collinrm@squareup.com>