pfctl: Warn users when they run into kernel limits

[kp]

Summary

Warn users when they try to add/delete/modify more items than the kernel
will allow.

[allanjude]

Reviewed By: allanjude

[donner]

Reporting an "error too many tables" is useful for adding a new table.
It's wrong for deleting and misleading for obtaining statistics.

[kp]

In D22733#497518, @lutz_donnerhacke.de wrote:

Reporting an "error too many tables" is useful for adding a new table.
It's wrong for deleting and misleading for obtaining statistics.

Good point. I'll see if I can improve the error message.

[donner]

In D22733#497656, @kp wrote:

In D22733#497518, @lutz_donnerhacke.de wrote:

Reporting an "error too many tables" is useful for adding a new table.
It's wrong for deleting and misleading for obtaining statistics.

Good point. I'll see if I can improve the error message.

I think we need different error messages.
Deleting a table might be stopped by "table in use".
Obtaining statistics might be prevented by "table not found".

If the error message does not reflect the real cause, it will create more harm, than omitting it.

[kp]

Improve error messages

[donner]

Thank you.