NetworkPkg: Fix PXEv6 boot failure when DhcpBinl offer received.

In case of the DHCP and PXE services on different servers,PXEv6 boot will
failure when DhcpBinl offer received. The issue is caused by the following
reasons:
* PXE Client doesn't append VENDOR_CLASS request parameter, so the
offer replied from DHCP service will not contain VENDOR_CLASS option
(16).
* Once the DhcpBinl offer is selected, the boot discover message should
be sent out to request the bootfile by this offer. Current implementation
always use servers multi-cast address instead of BootFileUrl address in
dhcp6 offer. we should check it first, then decide whether use multi-cast
address or not.
* If DhcpBinl offer is selected, the boot discover message shouldn't
find server ID Option from DhcpBinl offer. That's incorrect because DHCP
service and PXE service on different servers. In such a case, we can ignore
the Server ID Option.

With the above fix in the patch, PXEv6 can boot successfully when DhcpBinl
offer received.

Cc: Ye Ting <ting.ye@intel.com>
Cc: Fu Siyuan <siyuan.fu@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Wu Jiaxin <jiaxin.wu@intel.com>
Reviewed-by: Fu Siyuan <siyuan.fu@intel.com>
Reviewed-by: Ye Ting <ting.ye@intel.com>