agetty: always read additional issue file locations

agetty: always read additional issue file locations

Currently, additional locations (such as /run or /usr/lib) are only
read if /etc/issue does not exist. This behavior makes it difficult to use
local settings (/etc) together with generated (/run) and distribution-specific
(/usr/lib) files. Let's read all the locations.

If the output of issue files is unwanted, the option --noissue can be
used, or the default locations can be ignored by using --issue-file
<list>.

Fixes: https://github.com/util-linux/util-linux/issues/3279
Addresses: https://github.com/systemd/systemd/issues/35071
Signed-off-by: Karel Zak <kzak@redhat.com>