Add default values for new parameters
Add env whitelist/blacklist to svnfs

Also simplify how per-repo configuration is handled, as has been done in
gitfs, hgfs, and minionfs.
Add new params to master config docs

Also fixed up some of the wording on the other recently-added remote
fileserver configuration parameters.
Merge pull request #11927 from terminalmage/svnfs-env-whitelist

Add env whitelist/blacklist to svnfs