Added real back-link searches.

Added real back-link searches.
The pgsql and msql backends are completely untested at this point,
but they "should work".  (I.e.: they're probably broken now.)

The msql back-link search is a bit buggy, and will not find links
if they are split between two records ("lines") in the page store.
(The proper remedy for this is to implement the links table in
the msql back end.)

git-svn-id: svn://svn.code.sf.net/p/phpwiki/code/branches/release-1_2-branch@535 96ab9672-09ca-45d6-a79d-3d69d39ca109