ifcfg-rh: support serializaing all possible values for ethernet.s390-options (OPTIONS)

While the keys of s390-options are from a well-behaving set of names
(that is enforced by nm_connection_verify()), the values are arbitrary
strings.

Our settings plugin must be able to express all values of a connection,
hence we need to support escapes.
12 jobs for th/cli-team-cleanup in 32 minutes and 29 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Test
passed #259552
allowed to fail
checkpatch

00:01:31

passed #259559
t_debian:9

00:10:25

passed #259561
t_debian:sid

00:12:17

manual #259560
allowed to fail manual
t_debian:testing
passed #259553
t_fedora:28

00:24:49

passed #259554
t_fedora:29

00:32:27

failed #259555
t_fedora:30

00:00:52

manual #259556
allowed to fail manual
t_fedora:rawhide
passed #259557
t_ubuntu:16.04

00:10:16

passed #259558
t_ubuntu:18.04

00:12:45

 
  External
failed NetworkManager test verification

failed NetworkManager test verification

06:14:30

 
Name Stage Failure
failed
t_fedora:30 Test
$ date '+%Y%m%d-%H%M%S'; NM_INSTALL="dnf install -y" ./contrib/fedora/REQUIRED_PACKAGES
20190424-110826
Fedora Modular 30 - x86_64 3.0 MB/s | 2.1 MB 00:00
Fedora Modular 30 - x86_64 - Updates 320 B/s | 257 B 00:00
Fedora Modular 30 - x86_64 - Test Updates 4.2 MB/s | 2.9 MB 00:00
Fedora 30 - x86_64 - Test Updates 16 MB/s | 16 MB 00:01
Fedora 30 - x86_64 - Updates 431 B/s | 257 B 00:00
./contrib/fedora/REQUIRED_PACKAGES: line 13: 11 Segmentation fault (core dumped) $NM_INSTALL "$@"
ERROR: Job failed: exit code 1