dhcp: store dhclient's pid file in /var/run/NetworkManager instead of /var/run

The pid-file is private to NetworkManager. It should reside in NetworkManager's
run directory instead of "/var/run".

I don't think that changing this location can break existing uses. Why
would somebody outside of NetworkManager care about this file?
12 jobs for th/dhclient-pid-file in 59 minutes and 47 seconds (queued for 1 second)
Status Job ID Name Coverage
  Test
passed #306594
allowed to fail
checkpatch

00:00:56

passed #306601
t_debian:9

00:23:59

passed #306603
t_debian:sid

00:18:02

passed #306602
allowed to fail manual
t_debian:testing

00:16:11

passed #306595
t_fedora:28

00:25:57

passed #306596
t_fedora:29

00:33:56

passed #306597
t_fedora:30

00:24:16

passed #306598
allowed to fail manual
t_fedora:rawhide

00:25:50

passed #306599
t_ubuntu:16.04

00:19:46

passed #306600
t_ubuntu:18.04

00:24:09

 
  External
passed NetworkManager test verification

passed NetworkManager test verification

06:00:13