Work out the size of the buffer we need to read netlink messages as recent 64-bit Linux kernels have increased their header size and overflowed the fixed buffer we used.
Work out the size of the buffer we need to read netlink messages as recent
64-bit Linux kernels have increased their header size and overflowed
the fixed buffer we used.

Could also fix #192.

rsmarples/dhcpcd,rsmarples/dhcpcd,rsmarples/dhcpcd