MdeModulePkg: Update NetLib interface to support classless addressing.

MdeModulePkg: Update NetLib interface to support classless addressing.

The classful addressing (IP class A/B/C) has been deprecated according to
RFC4632. This patch updates the NetLib NetGetIpClass() and NetIp4IsUnicast()
accordingly.

NetGetIpClass()
The function is kept for compatibility, while the caller of this function
could only check the returned value against with IP4_ADDR_CLASSD (multicast)
or IP4_ADDR_CLASSE (reserved) now. The function has been updated to note this.

NetIp4IsUnicast()
The NetMask becomes a required parameter to check the unicast address.

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