networkd: bond support primary slave and active slave (#4873)

networkd: bond support primary slave and active slave (#4873)

active_slave:

Specifies the new active slave for modes that support it
(active-backup, balance-alb and balance-tlb).

primary slave:
systemd-networks currently lacks the capability to set the primary slave
in an
active-backup bonding. This is necessary if you prefer one interface
over the
other. A common example is a eth0-wlan0 bonding on a laptop where you'd
want to
switch to the wired connection whenever it's available.

Fixes: #2837