1. Sync Bug:PXE Boot issue- UefiPxeBc driver currently does not follow PXE Spec

1. Sync Bug:PXE Boot issue- UefiPxeBc driver currently does not follow PXE Spec
[Root Cause Analysis]
The PXE offer selection in the PXE driver is splitted into two passes and the two passes are required to follow the same flow. However, in current code, there is some difference between the two.
[Solution]
make the second offer selection pass exactly match the first one.

2. Fixed one bug when allocating pool for UDP transmit packet.

git-svn-id: https://edk2.svn.sourceforge.net/svnroot/edk2/trunk/edk2@5932 6f19259b-4bc3-4df7-8a09-765794883524