dhcp: include conditionals from existing dhclient configuration

Since commit 159ff232 ('dhcp/dhclient-utils: skip over
dhclient.conf blocks') we skip blocks enclosed in lines containing '{'
and '}' because NM should ignore 'lease', 'alias' and other
declarations. However, conditional statements seem useful and should
not be skipped.

https://bugzilla.redhat.com/show_bug.cgi?id=1758550
16 jobs for bg/dhcp-conditionals-rh1758550 in 27 minutes and 33 seconds (queued for 2 seconds)
latest
Status Job ID Name Coverage
  Test
passed #704691
allowed to fail
checkpatch

00:01:03

manual #704697
allowed to fail manual
t_centos:7.5.1804
passed #704698
t_centos:7.6.1810

00:24:34

passed #704704
t_debian:10

00:15:55

passed #704703
t_debian:9

00:15:54

passed #704706
t_debian:sid

00:18:52

manual #704705
allowed to fail manual
t_debian:testing
manual #704692
allowed to fail manual
t_fedora:28
passed #704693
t_fedora:29

00:25:47

passed #704694
t_fedora:30

00:27:31

passed #704695
t_fedora:31

00:27:22

manual #704696
allowed to fail manual
t_fedora:rawhide
passed #704699
t_ubuntu:16.04

00:20:09

passed #704700
t_ubuntu:18.04

00:15:56

manual #704702
allowed to fail manual
t_ubuntu:devel
manual #704701
allowed to fail manual
t_ubuntu:rolling