interfaces: move "mangle" function for bond slaves

This will allow it to be used by other OS.
interfaces: abstract a bit MAC address mangling for bonding devices

A variable in `struct lldpd_hardware` control the mangling. On the
other end, `hardware->h_ops->send()` should not be called directly
anymore. Instead, `interfaces_helper_send()` should be used instead.
interfaces: mangle source MAC address on bonds for other OS too