ip-address: Support filtering by slave type, too

ip-address: Support filtering by slave type, too

This patch allows to query all interfaces enslaved to a bridge or bond
using the following syntax:

| ip addr show type bridge_slave

Filtering has to be done in userspace since the kernel does not support
filtering on IFLA_INFO_SLAVE_KIND.

Functionality introduced in this patch is not fully complete since it
does not allow to match on type and slave type at the same time, but it
doesn't prevent implementing a dedicated slave_type match, either.

Signed-off-by: Phil Sutter <phil@nwl.cc>