agetty: Reprompt and reprint /etc/issue when asked

Add an 'agetty --reload' command which asks all running agetty
commands to display their prompts again.

Several of the /etc/issue escape codes such as \4 and \S depend on
variable data which can change after the agetty prompt is displayed.
This can cause stale data to be displayed when a user looks at a VT,
especially in cases of DHCP racing with system start up.

We never want this to occur once the user has started typing a
user name. So we detect when the user starts typing, after which
no further reprompting occurs after that point.

[kzak@redhat.com: - add #ifdefs to make it usable on non-inotify systems,
                  - use futimens() with NULL timespec
                  - add --reaload to usage()]

Signed-off-by: Stef Walter <stefw-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>
Signed-off-by: Karel Zak <kzak@redhat.com>