Force load of target shared object before major version check

The load tries to ensure it's the load of the _old_ library that fails,
rather than the one of the _new_ library.

A failed load of the old library would make major version check resort
to call the postgis_scripts_installed() which would contain the correct
version since version 1.1.0 up. Load of the old library should only fail
when the old library is 2.1.0 up so there's no problem there.

NOTE: Failure of loading the _new_ library could still happen if the upgrade
script is loaded in a session in which the _old_ library was used already.
This is not something we can fix as there's no "unload" command in PostgreSQL.

Closes #2382

git-svn-id: http://svn.osgeo.org/postgis/trunk@12641 b70326c6-7e19-0410-871a-916f4a2858ee