logging: use stack allocated string buffer to constuct NM_LOG_DOMAINS field

NM_LOG_DOMAINS is a comma-separated list of the selected logging domains.

As the number of all logging domains is fixed at compile-time, the maximum
length of the buffer is known.

  $ git grep $'^\t{ LOGD_' | sed 's/.*"\(.*\)" .*/\1/' | xargs | sed 's/  */,/g' | sed 's/^/NM_LOG_DOMAINS=/'
  NM_LOG_DOMAINS=PLATFORM,RFKILL,ETHER,WIFI,BT,MB,DHCP4,DHCP6,PPP,WIFI_SCAN,IP4,IP6,AUTOIP4,DNS,VPN,SHARING,SUPPLICANT,AGENTS,SETTINGS,SUSPEND,CORE,DEVICE,OLPC,INFINIBAND,FIREWALL,ADSL,BOND,VLAN,BRIDGE,DBUS_PROPS,TEAM,CONCHECK,DCB,DISPATCH,AUDIT,SYSTEMD,VPN_PLUGIN,PROXY

Note that the static buffer "_all_logging_domains_to_str" is known
to be large enough to contain these domain names (it's even longer,
as it also contains "ALL", "IP", and "DHCP" alises). We can use that
to define the array of suitable size.
12 jobs for th/logging-syslog-facility in 39 minutes and 54 seconds (queued for 1 second)
Status Job ID Name Coverage
  Test
passed #300868
allowed to fail
checkpatch

00:01:03

passed #300875
t_debian:9

00:15:02

passed #300877
t_debian:sid

00:12:47

manual #300876
allowed to fail manual
t_debian:testing
passed #300869
t_fedora:28

00:20:21

passed #300870
t_fedora:29

00:39:53

passed #300871
t_fedora:30

00:25:05

manual #300872
allowed to fail manual
t_fedora:rawhide
passed #300873
t_ubuntu:16.04

00:16:58

passed #300874
t_ubuntu:18.04

00:15:24

 
  External
passed NetworkManager test verification

passed NetworkManager test verification

06:20:33