Add the ability to build a release from an SVN checkout instead of a CVS

Add the ability to build a release from an SVN checkout instead of a CVS
checkout.  If SVNROOT is specified, then the source tree will be checked
out from that SVN repository instead of using CVS.  ports and docs still
use CVS.  If SVNROOT is not specified, then the source tree will be checked
out using CVS.  An explicit SVN branch can be specified using SVNBRANCH
(e.g. SVNBRANCH=stable/8).  If SVNBRANCH is not set but RELEASETAG is set
to a CVS branch (such as RELENG_8) the appropriate SVN branch will be
inferred from the CVS branch using svnbranch.awk.

Note that there are still several open questions about using SVN instead
of CVS in the release process.  However, this does enable one to build a
release from an SVN repository if needed.

Approved by:	re (kensmith)