dhcp: nettools: fix parsing of search domains option

'first' was never modified and so the dot was never added.

Fixes: 6adade6f ('dhcp: add nettools dhcp4 client')

https://bugzilla.redhat.com/show_bug.cgi?id=1783981
(cherry picked from commit 9aa00a8a)
(cherry picked from commit ea221353)
16 jobs for nm-1-20 in 32 minutes and 9 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Test
passed #1177279
checkpatch

00:00:52

manual #1177285
allowed to fail manual
t_centos:7.5.1804
manual #1177286
allowed to fail manual
t_centos:7.6.1810
manual #1177292
allowed to fail manual
t_debian:10
manual #1177291
allowed to fail manual
t_debian:9
manual #1177294
allowed to fail manual
t_debian:sid
manual #1177293
allowed to fail manual
t_debian:testing
manual #1177280
allowed to fail manual
t_fedora:28
manual #1177281
allowed to fail manual
t_fedora:29
passed #1177282
t_fedora:30

00:32:08

manual #1177283
allowed to fail manual
t_fedora:31
manual #1177284
allowed to fail manual
t_fedora:rawhide
manual #1177287
allowed to fail manual
t_ubuntu:16.04
manual #1177288
allowed to fail manual
t_ubuntu:18.04
manual #1177290
allowed to fail manual
t_ubuntu:devel
manual #1177289
allowed to fail manual
t_ubuntu:rolling