NetworkPkg: UefiPxeBcDxe: Add EXCLUSIVE attribute when opening SNP protocol installed...

NetworkPkg: UefiPxeBcDxe: Add EXCLUSIVE attribute when opening SNP protocol installed by PXE.

Ref: https://bugzilla.tianocore.org/show_bug.cgi?id=1152

v2: Sync the same logic to Ipv6 and update code comments.

The PXE driver installs a SNP and open this SNP with attribute BY_DRIVER
to avoid it being opened by MNP driver, this SNP is also expected not to
be opened by other drivers with EXCLUSIVE attribute. In some cases, other
drivers may happen to do this by error, and thus cause a system crash.
This patch adds EXCLUSIVE attribute when opening SNP in PXE driver, and
will reject all OpenProtocol requests by EXCLUSIVE.

Cc: Subramanian, Sriram <sriram-s@hpe.com>
Cc: Ye Ting <ting.ye@intel.com>
Cc: Fu Siyuan <siyuan.fu@intel.com>
Cc: Wu Jiaxin <jiaxin.wu@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Wang Fan <fan.wang@intel.com>
Reviewed-by: Sriram Subramanian <sriram-s@hpe.com>
Reviewed-by: Fu Siyuan <siyuan.fu@intel.com>
Reviewed-by: Wu Jiaxin <jiaxin.wu@intel.com>