Update files for the 1.7.1 release.
Revert "UpdateChecker: work around git-describe issue"

This reverts commit 3871c070cf54b41f0ac834b1166b8f7a511e5ee8.
Set MPC_BETA_RELEASE to 1.
Update Release.txt.
CID 966234.
CID 966348.
CID 966060.
Update to astyle v2.04.
Update Unrar to v5.0.0.13.
Remove DX SDK prerequisite.