bsd: Fix unimplemented copystr() and copyinstr()

Over the process of creating a zpool, history_str_get() will be eventually
called by zfs_ioc_pool_create(). history_str_get() will not work as expected as
it fully relies on the function copyinstr() which is currently unimplemented.

After that, spa_create() will be called with history_str storing trash, thus
making the spa_history_log filled with completely wrong entries.

Let's fix this problem simply by implementing copyinstr. By the way, copystr
has basically the same semantics as copyinstr, so let's implement it as well.
It's not being used anywhere, but who knows? It might be needed in the future
by some bsd compliant application.

As far as I know, this problem didn't manifest *visibly* anywhere as the spa
history log wasn't intentionally used by us yet. I found the problem while
auditing code.

Signed-off-by: Raphael S. Carvalho <raphaelsc@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>