[dhcp] Accept ProxyDHCP replies of type DHCPOFFER

The PXE standard provides examples of ProxyDHCP responses being encoded both
as type DHCPOFFER and DHCPACK, but currently we only accept DHCPACKs. Since
there are PXE servers in existence that respond to ProxyDHCPREQUESTs with
DHCPOFFERs, modify gPXE's ProxyDHCP pruning logic to treat both types of
responses equally.

Signed-off-by: Marty Connor <mdc@etherboot.org>

src/net/udp/dhcp.c

 	/* Filter out unacceptable responses */
 	if ( peer->sin_port != dhcp->proxy_port )
 		return;
-	if ( msgtype != DHCPACK )
+	if ( ( msgtype != DHCPOFFER ) && ( msgtype != DHCPACK ) )
 		return;
 	if ( server_id.s_addr /* Linux PXE server omits server ID */ &&
 	     ( server_id.s_addr != dhcp->proxy_server.s_addr ) )