Revert the change from commit 7b354baa (CMakeVersion: Set
CMake_VERSION_RC to 0 even in non-rc versions, 2019-07-25) and instead
define a `0` value in `CMake_VERSION_RC` to mean `-rc0`.  This
distinguishes release branch versions prior to the first release
candidate from the first release candidate itself.  It also makes room
for a dedicated "CMake $major.$minor.0-rc1" release commit for `-rc1` as
we have for later release candidates and final releases.