ifcfg-rh: add allow_filename_cb() argument to write-ifcfg-rh function

The function determines the filename automatically, but we
need to blacklist certain names.

That is, because NetworkManager keeps a list of loaded files
in memory. When writing a new file, we really want to choose
a filename that is not yet taken. For that we must not only
consider files on disk, but also files that existed on the last
time of loading.
13 jobs for th/various-settings-cleanup-3 in 40 minutes and 42 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Test
passed #377958
allowed to fail
checkpatch

00:02:31

manual #377963
allowed to fail manual
t_centos:7.5.1804
passed #377964
t_centos:7.6.1810

00:25:04

passed #377967
t_debian:9

00:20:42

passed #377969
t_debian:sid

00:19:07

manual #377968
allowed to fail manual
t_debian:testing
passed #377959
t_fedora:28

00:34:16

passed #377960
t_fedora:29

00:36:50

passed #377961
t_fedora:30

00:27:12

manual #377962
allowed to fail manual
t_fedora:rawhide
passed #377965
t_ubuntu:16.04

00:20:28

passed #377966
t_ubuntu:18.04

00:20:37

 
  External
canceled NetworkManager test verification

17:13:49