Automatically calculate the kernel revision for release tags by extracting

Automatically calculate the kernel revision for release tags by extracting
the revision from the tag itself.  Remove any DragonFly_ prefix or _Slip
suffix from the branch name.  This will allow us to create a slip tag that
synchronizes the sources with the automated subversion increment.  This also
reduces the work required to create a new, properly identified branch.

Automatically postpend a subversion from /usr/src/sys/conf/subvers-TAG, on
release builds only, where TAG is in the form RELEASE_X_Y.