agetty: handle systems without a shell or /bin/login

agetty: handle systems without a shell or /bin/login

When the login program (default /bin/login, or the one set with
--login-program) is missing or not executable, logging in is impossible.
Previously agetty still displayed the issue file and prompted for a
username, producing a confusing dead end.

Before prompting, check whether the login program is executable with
access(X_OK). If it is not, print a short banner ("This system does not
permit logins." by default, configurable with --nologin-message) and
wait for the user to press Enter, then re-check so an administrator who
installs the login program at runtime can proceed without a reboot. The
normal prompt flow is unaffected when the login program exists.

Add --nologin-message to override the banner text and document the new
behavior in agetty.8.adoc.

Closes #4117