agetty: Reload only if it is really needed

agetty: Reload only if it is really needed

If netlink event arrives and related escapes are part of issue, agetty
reloads and re-display the prompt. Reload is triggered not only by IP
address change, but also by IPv6 RAs. In some environments it causes
reload several times in a minute, and even complicates the login.

To prevent this, reload only if a real change appears.

This consists of:
split print_issue_file() to several functions:

eval_issue_file() prints issue to memory. It does not affect terminal in
any way.

print_issue_file() prints issue file from memory.

cmp_issue_file() compares the issue file and returns true, if reload is
needed.

The implementation requires additional change:

do_prompt() does not evaluate the issue file. It is responsibility of
calling function.

Test suite:

Use issue that contais \4 and/or \6 escape.

After installing new instance, restart agetty by typing a letter and then
Enter 6 times.

To check whether reload happens, type a letter. When reload happens,
letter disappears.

1. Unplug network cable. Wait a while and re-plug network cable.
You should see 2 reloads on single stack and 3 reloads on dual stack.

2. Run a loop
while : ; do
	sed -i '$areload_test' /etc/issue
	agetty --reload
	sleep 3
	sed -i '/reload_test/d' /etc/issue
	agetty --reload
	sleep 3
done
You should see regular reload every 3 seconds.

3. Run a loop
while : ; do
	agetty --reload
	sleep 3
done
Before: You see regular reload every 3 seconds.
After: No reloads.

4. Run a loop
while : ; do
	ifconfig lo 127.0.0.1 netmask 255.0.0.0
	sleep 3
	ifconfig lo 127.0.0.2 netmask 255.0.0.0
	sleep 3
done
Before: You see regular reload every 3 seconds.
After: No reloads.

Signed-off-by: Stanislav Brabec <sbrabec@suse.cz>