Revise versioning system to be date-based.

I'm going through all my projects and reworking them to avoid
depending on the monotonic integer-valued source control revision
identifier provided by Subversion, so I can migrate everything to git
without my builds and versioning breaking.

agedu's version number is now of the form YYYYMMDD.vvvvvv, where
vvvvvv is some string of source control information (currently still
the SVN-style "rNNNNN", but free to change in future). The date
provides monotonicity between my official automated builds, and the
second component is the one I'll be most interested in when people
send bug reports.


git-svn-id: svn://svn.tartarus.org/sgt@10244 cda61777-01e9-0310-a592-d414129be87e