uscan: support direct access to Subversion repositories

In this MR for uscan I have implemented mode=svn, direct access mode to Subversion repositories in a very similar way as mode=git is implemented. My motivation is that some packages (look up 'svn export' in d/orig-tar.sh) have their releases in Subversion repositories only. As with mode=git, versionless mode is supported also.

Examples of use:

1. Looks under ^/tags for most recent version:

version=4
opts="mode=svn, pgpmode=none" \
 svn://svn.code.sf.net/p/jmol/code/tags/ \
 ([\d.]+)\/ debian uupdate

2. Versionless (aka HEAD) mode:

version=4
opts="mode=svn, pgpmode=none" \
 svn://svn.code.sf.net/p/jmol/code/trunk/ \
 HEAD debian uupdate

Comments are welcome. Related BTS bug is 939538.