ch8/ch8-mac: Clear Unicast/Multicast bit

[ewenmcneill]

To match MAC address specification (eg, as summarised at https://en.wikipedia.org/wiki/MAC_address#Address_details) and Figure 8.4 of the text, clear the LSB of the 1st octet when generating the MAC address, rather than setting it. Also update check for "is_unicast()" to match.

Equivalent fix (from 2020-08-13) in the ch8-mget example: #7 (comment), which seems to have already been fixed when the first edition of the book was released (7d7955e):

code/ch8/ch8-mget/src/ethernet.rs

Lines 23 to 31 in 6ebd519

	impl MacAddress {
	pub fn new() -> MacAddress {
	let mut octets: [u8; 6] = [0; 6];
	rand::thread_rng().fill_bytes(&mut octets); // <1>
	octets[0] |= 0b_0000_0010; // <2>
	octets[0] &= 0b_1111_1110; // <3>
	MacAddress { 0: octets }
	}
	}

but apparently ch8/ch8-mac did not get fixed at the same time.