Add zfree to zero allocation before free

Key and cookie management typically wants to
avoid information leaks by explicitly zeroing
before free. This routine simplifies that by
permitting consumers to do so without carrying
the size around.

Reviewed by: jeff@, jhb@
MFC after: 1 week
Sponsored by: Rubicon Communications, LLC (Netgate)
Differential Revision: https://reviews.freebsd.org/D22790