New version scheme to support branchy workflow

Prepare to switch to the workflow described by "git help workflows".  In
this workflow, the "master" branch is always used to integrate topics
ready for release.  Brand new work merges into a "next" branch instead.
We need a new versioning scheme to work this way because the version on
"master" must always increase.

We no longer use an even/odd minor number to distinguish releases from
development versions.  Since we still support cvs checkout of our source
tree we cannot depend on "git describe" to compute a version number
based on the history graph.  We can use the CCYYMMDD nightly date stamp
to get a monotonically increasing version component.

The new version format is "major.minor.patch.(tweak|date)".  Releases
use a tweak level in the half-open range [0,20000000), which is smaller
than any current or future date.  For tweak=0 we do not show the tweak
component, leaving the format "major.minor.patch" for most releases.
Development versions use date=CCYYMMDD for the tweak level.  The
major.minor.patch part of development versions on "master" always
matches the most recent release.

For example, a first-parent traversal of "master" might see

        v2.8.1      2.8.1.20100422    v2.8.2
           |              |              |
  ----o----o----o----o----o----o----o----o----

Since the date appears in the tweak component, the next release can
increment the patch level (or any more significant component) to be
greater than any version leading to it.  Topic branches not ready for
release are published only on "next" so we know that all versions on
master lead between two releases.

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