Fix selfupdate-svn on Mac OS X 10.5 and svn 1.5

'--depth=files' is unsupported with 10.5's built-in svn-1.4.
Thus we use --non-recursive for older svn versions instead.
Initial text change.  More to follow.
git: Use Fink::Selfupdate::Base->devtools_check()
svn: Use Fink::Selfupdate::Base->devtools_check()
Don't check out the whole tree when updating.
Cleanup SelfUpdate::Base::devtools_check
Selfupdate: clarify some POD comments
Selfupdate: simplify code
Remove last traces of the long-obsolete SelfUpdateCVSTrees