Clean up common ifaddr management:

• Unify reference count and lock initialization in a single function, ifa_init().
• Move tear-down from a macro (IFAFREE) to a function ifa_free().
• Move reference count bump from a macro (IFAREF) to a function ifa_ref().
• Instead of using a u_int protected by a mutex to refcount(9) for reference count management.

The ifa_mtx is now used for exactly one ioctl, and possibly should be
removed.

MFC after: 3 weeks