agetty: add support for /etc/issue.d

agetty: add support for /etc/issue.d

The /etc/issue file has been originally designed to inform users
about the system (version, name, etc.).

In last years is growing number of additional tools (containers,
maintenance tools and interfaces, ...) and many admins and downstream
maintainer want to add some tool specific hints to the issue file, but
it mess to share one file between more packages and/or scripts. The
solution is /etc/issue.d directory.

The directory is extension to the standard system /etc/issue. The
/etc/issue file has to exist, otherwise the directory will be ignored.
It means "rm /etc/issue" (or --onissue) is still the way how keep our
system silent independently on 3rd-party installed files in the
/etc/issue.d directory.

The content of the files in the directory are printed after content of
the /etc/issue. The files are printed in version-sort order and .issue
file extension is required (00-foo.issue 01-bar.issue ...).

The change is backwardly compatible.

Signed-off-by: Karel Zak <kzak@redhat.com>