[RFC] Tools/ script for automatically making a tar out of svn sources

[Eitan Adler]

The attached script is designed to work with the 20+ ports that
currently have to resort to hacks to automatically figure out the head
version, checkout from svn, make a tar file, and then upload the file
to freefall.

It is based on some my earlier work/proposals
(http://wiki.freebsd.org/EitanAdler/ports-svn) to put this directly
into ports.*.mk. While that proposal was rejected by a large part of
the community making a simple standard script to put into Tools was
suggested by a few people as a better solution and one more likely to
get accepted by the community and portmgr.

This port requires that three values be defined in the ports Makefile.
Of these two are already defined for most of the ports that use the
hacks mentioned above.
USE_SCM="svn" is required as I plan on including support for other
common SCMs that might be used in the ports collection already (git
and cvs come to mind)
SVN_REV=12345 is required unless you use the "-h" option which gets
the version from head
SVN_URL=svn://goo.com/svn_repo - this is where the source is fetched from.

If I could get any comments (1) on the script in particular and (2) if
the approach I'm taking now is better than the one I tried a few weeks
ago (see wiki page) it would be really good.