Improve limit handling, and extend it to importing also.

Use ITEMS_MAX define in common.h instead directly using ULONG_MAX. However,
ITEMS_MAX is also defined as ULONG_MAX.

Be smarter when creating new keychains/keys, and don't delete the created
keychain/key after the fact, if the element limit was reached, but check beforehand.

Check limits during 'copy' and 'import*/append*'.

Remove Keychains and Items (all) outputs from 'status' command. Also, extract
count_items() from there, and use the new count_elements() (from commands.c)
where needed.

Bring documentation up to speed.