Symbolic links inside the DWiki page area now cause redirections if
their value would be a valid redirection in a REDIRECT line.

For example: FrobTig, which is a symlink in /Aliases with the value
'_../People_'.

We could have tried using os.path.realpath() on the symlink and taking
the result relative to the store root, but I think that that has more
subtle explosive breakages.

Symbolic links that don't resolve to a real DWiki page this way are
interpreted normally, so you can still have symlinks that point to
files outside the DWiki pagestore root.