networkd: fix kernel rtnl receive buffer overrun error

We got the following error when running systemd on a device  with many ports:

"rtnl: kernel receive buffer overrun
Event source 'rtnl-receive-message' returned error, disabling: No buffer space
available"

I think the kernel socket receive buffer queue should be increased. The default
value is taken from:
"/proc/sys/net/core/rmem_default", but we can overwrite it using SO_RCVBUF
socket option.

This is already done in networkd for other sockets.
For example, the bus socket (sd-bus/bus-socket.c) has a receive queue of 8MB.
In our case, the default is 208KB.

Increasing the buffer receive queue for manager socket to 512KB should be enough
to get rid of the above error.

[tomegun: bump the limit even higher to 8M]