agetty: extend --issue-file to support multiple paths

agetty: extend --issue-file to support multiple paths

The current default behavior is to print the first issue file/dir and
all alternative locations are used as a backup solution only. If something
is found than the rest is ignored. The --issue-file allow to overwrite
this default behavior, but currently it supports only one file/dir.

This patch extend --issue-file to support ':' separated list of paths
and *all* the files (if exist and no empty) in the list are printed.

 agetty --issue-file=/etc/issue:/etc/issue.d:/run/issue:/run/issue.d:/usr/lib/issue:/usr/lib/issue.d

Addresses: https://github.com/karelzak/util-linux/issues/1041
Signed-off-by: Karel Zak <kzak@redhat.com>